I was running into the same issue. Changing the permissions of the tools.php file doesn’t help. Here’s what I did: When you go to create a new user, log in to Instagram, and after you are logged it should redirect you back to your WordPress admin tools.php page with a 404 error. Copy the URL and all of the parameters that were assigned when you were brought back to WordPress. Remove all the parameters except access_token, full_name, profile_picture, and id, and then paste that URL back into my browser and try again. It worked for me.
