dissatisfied with one's assigned role; feeling the role is beneath one
Expresses that the speaker considers the given task or position too insignificant for their abilities. Often used in formal or workplace contexts. Note: This is the original, correct meaning; colloquial misuse as 'not up to the task' is common but considered incorrect.
He felt the job was beneath him and didn't accept it.
Such a simple task is not worthy of my abilities.
not up to the task; out of one's depth
Colloquial misuse of 役不足 to mean 'lacking ability for the role'. The correct term for this meaning is 力不足 (ちからぶそく). Despite being widespread, it is often flagged as an error in formal writing.
See also: 力不足 (ちからぶそく)
He's not up to the task, so we can't entrust the project to him.
I might be out of my depth, but I'll do my best.