No valid recipients resolved

We have a sent functionality having "Appian Group" in the To field. Group have users and it is working fine in our DEV and SIT environment.
But the problem is, not working and getting this error in UAT and PROD environment alone.

I gone through some existing threads and they suggested to use toemailrecipient() function. We didn't use, but it is working good in DEV and SIT. So, please suggest what could be teh problem with higher environments.

Problem:  An error occurred in executing an Activity Class.

Details:  No valid recipients resolved

Recommended Action:  Examine the activity class to correct the error and then resume.

Priority of this problem: High Priority