Krishna repeats a teaching He gave earlier in Chapter 3 (verse 35), and the repetition is deliberate. Your own dharma — even if you perform it imperfectly — is better than someone else's dharma performed with polish. And a person who does the work ordained by their own nature incurs no fault.
It is natural to look at another person's path and think: that looks easier, that looks more rewarding, that suits me better. But the Gita says otherwise. A fish trying to climb a tree will always feel like a failure, no matter how sincerely it tries. Its dharma is to swim. That is where its perfection lies.
The reassurance in the second line is quietly powerful: 'svabhavaniyatam karma kurvan napnoti kilbisham.' Doing the work that your nature assigns you does not generate sin — even if the results are imperfect. The alignment between nature and action is itself a form of purity.