After describing the rare, liberated sage who has no duty (Verses 17-18), Krishna brings the focus squarely back to Arjuna with the word 'Tasmāt' (Therefore). Because Arjuna is NOT yet that fully liberated sage, Krishna commands him: 'Asaktaḥ satataṃ' (remaining constantly unattached), you must properly perform (samācara) the work that is your prescribed duty (kāryaṃ karma). Why? Because a person who performs their duty completely unattached (asakto hy ācaran karma) inevitably attains the Supreme Destination (param āpnoti pūruṣaḥ).