Solution:
The error 403 caused by below reasons:
1. Plugins: temporarily deactivate all the plugins includes any security plugins that you may have installed on your site. activate all plugins one by one and you can figure out which plugin was causing the error.
2. Corrupt .htaccess: Download the .htaccess file to your computer so that you have a backup of it. After that, you need to delete the file from your server. You can generate new .htaccess file from your WordPress admin area, go to Settings -> Permalinks page and just save the changes.
3. Wrong File Permissions: All folders on your WordPress site should have a file permission of 744 or 755. and All files should have a file permission of 644 or 640.
If all things are correct on your website, ask your server provider to help you with the issue.