WordPress Issue: Cannot modify header information - headers already sent by

This error message appears when a PHP script is trying to modify the headers of an HTTP response after the headers have already been sent. This can happen if there is whitespace or other output before the call to the header() function, or if there is a PHP error or warning message being output to the browser. To fix this issue, you'll need to find the source of the output and remove it. This could be an echo statement, a print statement, a debugging message, or a plugin or theme that is not properly configured. You can also check for any blank spaces or unnecessary characters before the opening <?php tag in your files.