3 answers
Contacts that cannot receive messages (especially MMS) may simply be out of credit, or their SIM card is defective (or not inserted in a phone).
This happened to me with one contact:
when I tried to send them an MMS, I kept receiving the warning "the message was not sent because the service is not activated."
But I could send the same MMS to other contacts without any problem.
One day later, I received a message from the concerned contact (with a different number) telling me that they had changed their phone (and number).
To make it simple, this seems to happen when you try to send an MMS to a number that has become "ghost" (which is no longer assigned).
If this can help someone...
Me too, but on just one.