When you click on the "Access" button, dbFront attempts to reach out to your authentication source to retrieve a list of valid users.
That error message means that dbFront was not able to connect to your authentication source. You can check your error log by opening [Help]/[Log Viewer] to see if you can find some more details.
If there are not enough details to help then you can enable extra debugging information. Specifically you might want to enable the DebugSecurity flag. For instructions see: Debugging Help