Error when trying to active a new plugin

Warning: Cannot modify header information – headers already sent by (output started at /websites/123reg/LinuxPackage21/ma/ke/be/makebelievegroup.co.uk/public_html/sitedev/wp-includes/formatting.php:1) in /websites/123reg/LinuxPackage21/ma/ke/be/makebelievegroup.co.uk/public_html/sitedev/wp-includes/pluggable.php on line 1196

Warning: Cannot modify header information – headers already sent by (output started at /websites/123reg/LinuxPackage21/ma/ke/be/makebelievegroup.co.uk/public_html/sitedev/wp-includes/formatting.php:1) in /websites/123reg/LinuxPackage21/ma/ke/be/makebelievegroup.co.uk/public_html/sitedev/wp-includes/pluggable.php on line 1196

This is my error, I don’t know what I can try.
Would be great if somebody could help me!
Thanks!