Nipe means 'give me', as in 'You' asking someone else to give you money. Not 'I'
Nipe is also the 1st person singular subjunctive of kupa without an object. It's ambiguous. Of course, this will generally be interpreted as "give me", because you usually say who you'd give money to, but this Duolingo course is full of incomplete sentences ... and the fact that this comes up in the subjunctive lesson, rather than the imperative lesson predisposes one to interpreting it as subjunctive.
But I think in the case of kupa you only use that verb if you know the object. So ni- must be the object, not subject. ?
I'm not understanding why Duolingo presents this conjugation as the subjunctive in its basic form. Combined with other verbs, it can function as a subjunctive, but by itself, all my other books and practical experience have this as a polite command form, or a suggestion, ie "Let's" Any time I've heard "Nipe," it is "give me." It does seem the conjugation would be the same, though. But maybe it needs an object... ie I should give them money, niwape pesa?
"Kutoa" means to offer, produce or evoke, not directly give. Kupa is ambiguous in the subjunctive, and as @AGreatUserName has said, it could mean both.
Why doesn't nipe pesa mean "I should give money"? Seems perfectly possible to me.