"I can buy everything!"
Translation:Mogę kupić wszystko!
Both are fine actually. Since Polish is based a lot on inflection the actual word order in most cases does not matter, the word endings make the meaning and word relation. Just that it does not sound "natural" when it is changed. I guess the correct answer is usually the most common way of saying it. Nonetheless I can see someone saying both "mogę kupić wszystko" and "mogę wszystko kupić", and both mean the same.
I keep getting marked wrong for putting the object before the verb in this sentence but that order is possible in other sentences and the comments here seem to indicate it should be accepted , and is even more natural. Please allow it or explain why it is not accepted.
I'd say that's significant in imperative (Do it/Don't do it) when indeed it's usually imperfective in negations, but here, not that much. It's rather dependent on what exactly you mean or how you interpret the sentence.
I just added "kupować" here, it seems possible, although somehow less likely - it would mean that I am able to buy everything (I want) whenever I want, on many occasions, on many trips to the store.
"Nie mogę kupować wszystkiego" put that interpretation forward, like "I am generally too poor to buy everything (that I want)". But "kupić" would also work, if you meant a specific occasion, like "I cannot buy everything that I was supposed to buy today". Or maybe actually "I'd like to own everything but I can't buy it".