You can see the plugin directory paths in WooCommerce > PDF Invoices > Advanced > Status > Directory Permissions (located at the bottom). There, you can also check the current permissions for each directory.
Please note that you can change the file system method, if you need it, in WooCommerce > PDF Invoices > Advanced > Settings > File System Method:
It looks like your server does not have the PHP FTP extension enabled.
Could you please enable the PHP FTP extension for your website? You can usually do this in your hosting control panel under PHP settings or “Select PHP Extensions”.
If you’re not sure how to do this, please contact your hosting provider and ask them to enable the PHP FTP extension (ext-ftp or ftpext) for your account.
Once it’s enabled, please try opening a generated PDF invoice again to check that everything works correctly.