नियतस्य तु संन्यासः कर्मणो नोपपद्यते। मोहात्तस्य परित्यागस्तामसः परिकीर्तितः ॥

niyatasya tu sannyāsaḥ karmaṇo nopapadyate mohāt tasya parityāgas tāmasaḥ parikīrtitaḥ

Verily renunciation of actions that are prescribed is not proper; the relinquishment thereof from delusion is said to be of darkness.

In plain words: Krishna says it is wrong to give up the duties you are meant to do. Dropping them out of confusion is the lowest, darkest kind of giving up.