Not Logged in User Couldn’t upload image

Most likely you are using some kind of security plugin which is preventing access to the wp-admin/admin-ajax.php page, please go to wp-admin / Plugins and try disabling the plugins one by one and see if the problem goes away.

If you know which plugin does this maybe it is possible to make an exception for the wp-admin/admin-ajax.php path as this page should actually be accessible to both logged-in and not-logged-in users.