Norway is definitely not cashless and there are plenty of ATMs around. That said, stores have been requesting people not use cash during covid, so that plays a big role. You can't miss the cash registers in stores, though.

If your card is a debit card, you might have to talk to the cashier about specifically setting the reader for a credit card transaction. Debit cards do run through a separate process, and your US card would fail. But keep in mind that the same thing happens with Norwegian cards in the US - debit cards need to be run as credit otherwise they'll fail.