mPDF Error

Hello Guys! I Just Installed DK-PDF on my local machine to test it so I can start a project with it soon. I created a sample page so I could test the conversion to PDF through the PDF button. However, when I click on the button, I get the following error:

mPDF error: Some data has already been output to browser, can't send PDF file

I’m quite lost here so would love to get some help. I’m using DK-PDF version 1.9.3 and PHP version 7.2.0. Here’s the full stack trace in case you need it:

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: A non-numeric value encountered in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\mpdf.php on line 31965

Warning: A non-numeric value encountered in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\mpdf.php on line 31965

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: A non-numeric value encountered in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\mpdf.php on line 31965

Warning: count(): Parameter must be an array or an object that implements Countable in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\otl.php on line 5377

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1136

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1139

Warning: Illegal string offset 'ID' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Illegal string offset 'LANG' in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php on line 1190

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php:1139) in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\mpdf.php on line 8314

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\classes\cssmgr.php:1139) in C:\xampp\htdocs\wordpress\wp-content\plugins\dk-pdf\includes\mpdf60\mpdf.php on line 1706
mPDF error: Some data has already been output to browser, can't send PDF file