This article covers methods to fix Pluggable.php File Errors in WordPress. Basically, this WordPress error triggers when we add a code snippet on our site or activate a new plugin.
This means that Pluggable.php file errors takes place as a result of a poorly coded theme or plugin. It could also be a result of code errors in your WP files.
The first option when troubleshooting the Warning: cannot modify header information pluggable.php error is to manually edit the faulty file. You are already supplied with the necessary information to locate the problem in the error message (remember, it's the first file in the message).
All you need to do is open up this file, either through an FTP client like FileZilla, or through File Manager.
Make sure to pay special attention to the starting and ending PHP tags.
There should not be spaces before or after the <?php tag, as well as the ?> tag.
Also, the last line of code should not be followed with a blank line or extra space.