I believe these are the instructions I found online that worked.
After software upgrade IOS 11.3 today I had this message appear also...
This works as I have just done it:
-Sign out of ICloud
-re boot your IPhone
-Sign back into ICloud
-Message has disappeared
I had to do it 2 times and now the message has disappeared but once you reboot go to your message first to see if that disable message has left if it has log back into IClound, If it hadn't reboot again before logging back into ICloud. Like I said I had to do it twice.