Appearance -> Editor

Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the default theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

– re-uploading all files & folders – except the wp-content folder and wp-config.php & root .htaccess files – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.

Bill here told me to edit a file when I had the same issue. Here is. His advice:

If you determine that you do have full admin capabilities, but the site was set up by someone else, you might also check your wp-config file for define(‘DISALLOW_FILE_EDIT’, true);
which is sometimes used as a security measure. Setting it to false will display the editor again. Many of us prefer to use other editors by default anyway.