In order to identify why are you receiving this error you must access the server in question and from a command prompt
C:\windows\system32 run > Type slmgr.vbs -ato (this activates the server)
Below you will find in this case why you are not able to activate the server.

Proceed to run > slmgr.vbs -ckms (this clears and KMS entry you may have)

3.Type slmgr.vbs -upk (this removes any product key installed)

4.Type slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x's is the product key you wish to use)

5.Type slmgr.vbs -ato (this activates the server)

After these steps everything is now showing as activated