This article helps you to understand, how to Fix WordPress Cannot modify header error.
This PHP header error results from extra empty spaces that are not suppose to be within your files. You could have abruptly created them when you are modifying WordPress or your Theme files.
1. Upload a Fresh Copy
When you read the error message and you determine that the broken file is a part of the WordPress core or it’s otherwise an inherent file of a plugin or theme, there’s a quick fix available.
If it’s a regular plugin or theme file, you can uninstall and reinstall it.
If you would rather not do this or a WordPress core file is affected, you can download a fresh copy and extract the ZIP archive to your computer. Locate the file that’s broken on your site and replace the existing one on your server with the fresh copy.
You can do this with the help of FTP, SSH or directly through cPanel.
2. Edit the Trouble File
Once you have identified the problem file, you can download a copy of it through FTP or SSH, or you can edit it directly in cPanel.
Clear any extra blank lines or spaces at the beginning and end of the document with your backspace button or Delete button for PC. Also, be sure there are no spaces before, in between or after the <?php tag at the beginning of the file. The same goes for the ?> tag at the end.If the final line of the document doesn’t end in a ?> tag, then be sure there are still no spaces after the final character of the last line. Save the file and upload it back to your site in the same place, overwriting the original file.