niyatasya tu saṃnyāsaḥ karmaṇo nopapadyate |mohāttasya parityāgastāmasaḥ parikīrtitaḥ ||
Giving up a prescribed duty is simply not proper. Abandoning it out of delusion is declared to be tamasic.
Explanation
Krishna is explaining the different kinds of renunciation (tyāga) to Arjuna. A prescribed duty (niyata karma), one that belongs to a person's role and responsibilities, should not be abandoned. Giving it up out of confusion (moha) is called tamasic, meaning it arises from the guna of darkness and inertia (tamas).