The requirements on callsign entry for WSPR are on page 4 of the manual (v2.03).?
WSPR will try to take your callsign from the "Callsign" setting and will make sure it satisfies the listed callsign requirements for WSPR. If the callsign is empty or invalid (doesn't satisfy the right format for WSPR) then the kit will try to use the "Message" parameter instead, in case you have put your callsign in there. If that is empty/invalid too then you will get that error message "Error: Message".?
In your case you must just enter "M0ELS", and as Andy says, NO space character at the end. (Unnecessary note: you could put a space character at the beginning, and it would be fine, i.e. " M0ELS". But if you do not, the kit will put one in automatically).
