This error indicates that there are some issues with your AJAX configuration respectively wp-admin/admin-ajax.php endpoint.
To verify this, please open the browser console (e.g. with F12 in Firefox or chrome), switch to the “Console” tab and reload the page and click on “Check migration” again to verify if there are any additional error messages:
If you do not see an error message there, please switch to the “Network” tab and reload the page and click on “Check migration” again to verify if there are any additional error messages:
From the screen above you see that the request failed with the Status (=HTTP error code) 404 – in this example, the file /wp-admin/admin-ajax.php was not available.
It is more likely though, that you will see Status 500 (= Internal Server Error) here. The 500 Internal Server Error is the general catch all error when the server throws an exception. It is the error message when no more specific error message is suitable and is in most cases directly related to a server misconfig or issue.
So what do now?
First, please try to temporarily disable all other plugins but Maps Marker Pro to verify that this issue is not caused by a plugin conflict.
If this does not help, please have a look at the server error log like PHP or Apache error log when that request is made and which might give you a clue why this error happens on your server.
If this still does not help, please check if one of the following tutorial can help you find the reasons for this server error:
If this still does not help, we´d advise to get in contact with your hosting support so that they can analyse why the request to /wp-admin/admin-ajax.php fails when trying to run the migration tool.