Thank you for reaching out. I can certainly understand your frustration with receiving error messages on his Galaxy watch 4. I was able to find some information on the AT&T forums regarding the error message. It appears the SIM either wasn't properly activated on the account, or the SIM numbers don't match on the account. The thread I found is regarding phones but would still also apply to the watch 4. Give this link a try for more assitance: https://forums.att.com/conversations/other-phones-devices/sim-card-error-saying-sim-not-provisioned-...
The next step would be to reach out to your carrier for further assistance.
