If an Angel of the Lord is incarnated in to Malkut to do a specific task and fails at that task, is it permissible for that Angel to take it’s own life?

Angels do not have the power of choice, like man has. Hence if an angel fails he hasn't the will nor the capability to choose by himself how to proceed, but these are things beyond our comprehension and are considered part of "Hanistarot Lashem Elokeinu" (the hidden is to G-d). Rabbi Ro'i Margalit