Hi Issa
The message in the program assignment is showing that there are some users with exceptions. This means that the user's completion date calculation has not been able to be calculated.
The exceptions can be fixed using bulk Actions as described here
The users should be notified with the program notification message as soon as they are enrolled in the program. Users with the exception messages aren't enrolled in the program until the exception has been fixed so they won't receive the program notification message until then.
regards