iMessage sign in issue. MacBook Pro OS 10.15.7
My mother-in-law has a Macbook Pro and is running OS 10.15.7 A week ago she was unable to send any messages through iMessage. I went over and logged her out of iMessage (found out she wasn't logged in) tried to log her back in and kept getting an activation error. So I signed her out of iCloud and restarted the laptop. Upon restart I signed her back into iCloud with no problem. Then went to iMessage, still no luck I got an activation error. I then attempted to sign into iMessage using my Apple ID and had no problem.
What else should I be looking at? Any ideas why she can't sign into iMessage. She also has a wifi only iPad that iMessage is working fine on.
Thanks for any help
MacBook Pro