When modifying an agent to change the agent group we are unable to given an error message:
no user defined group exists
no user defined group exists
DLP 15.X, 16.X
The agent group that was created does not have an agent configuration assigned to it.
If this has not resolved your issue, please see topic: Cannot select "Default Group" after moving DLP Agent out of Default Group
This topic is useful if you are unable to change agent groups in the enforce console when original group is 'default' or you are otherwise are having difficulty reassigning an agent group.