Hi, In this step by step video I will show you how you can fix disable add new plugin option in your WordPress website. Here are three possible ways to solve this issue
1. You need to deactivate all your plugins and check if that button is visible or not. If that is visible now then there is a problem with your plugins try to activate plugins one by one and find which plugin creating a problem.
2. Activate some other theme to check that your theme may be creating a problem. If your theme creating problem update it and if already updated find this code in your function.php file "DISALLOW_FILE_MODS" and remove this from your file.
3. If the above two tricks don't go to work then you need to replace all your WordPress core files with fresh files and make sure you have already created a backup of your files. Delete all files and folders except wp-content folder, wp-config.php file and your backup folder.
That's all. Hope so it will work.
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