Learn Farsi: این کار دو نتیجه دارد؛ یکی فایده است و دیگری عیب. - This action has two results; one is a benefit and the other is a flaw.

They are contrasting indefinite pronouns translating to "one" and "the other". The suffix -i at the end of yek (one) and digar (other) turns them from adjectives into standalone pronouns, so they effectively mean "one of them" and "the other one".

Why is the noun natije (result) singular when there are two of them (do natije)?

This is a strict rule in Farsi: whenever a noun is preceded by a specific number, the noun itself always stays in its singular form. You say do natije (literally "two result"), never do natije-ha.

Why isn't there a verb at the very end of the sentence after eyb (flaw)?

Farsi often drops the verb in the second half of a parallel sentence to avoid repetition. The full thought is ...va digari eyb ast (...and the other is a flaw), but the second ast (is) is simply understood from the first half.

Can kar translate to anything other than "work" or "job"?

Yes, kar is a very versatile word. While you probably first learned it as "job" or "work", it is also the standard word for an "action", "deed", or "activity". In this context, in kar means "this action" or "doing this".