Moving or copying tasks error with "User cannot be assigned to Role"
Selecting some tasks from a project and attempting to move them to another location in the same project, errors with "User cannot be assigned to Role"
Look at each task you're trying to move. Look for the user referenced in the error message.
Edit the task the erroring user is assigned to.
In the assignments area, make sure the user in question is assigned to a job role.
Repeating the move process may also error again with a different username. Repeat the steps above until the users each have a role assigned.