Controller MAC couldn't found


my installation on a Synology DS-916+ works fine. The controller is running and sync with designer was successful.

My problem is to add my controller in the designer.

When using the MAC Address i got an error:

No controller with the given MAC address was found or maybe it’s linked already.
Please start your controller before linking it, make sure it is not linked yet and that it has internet access.

What can i do?