How to solve this error?

The admin-ajax.php endpoint should work correctly for actual website visitors. However, the page seems to suggest that it may be doing a check to make sure the admin-ajax.php endpoint actually works, and that part is blocked. Most likely, nothing is actually preventing the plugin from working. The only question is whether the plugin will let you bypass this message or insist that it’s right and that you must fix a non-existent issue.