This is a limitation of the Asterisk PBX.
The phone is not registered to the extension, you can add the device anyway.
It may not be configured properly on the server, or not supported by phone.
The Asterisk Manager Interface is how Flexor communicates with the Asterisk server. It must be configured properly on the server (please click on this link to the guide).
Please click on this link to the Flexor Connect for Asterisk guide.