"We are paying."
We may not use "sind" (to be -- are) before the verb.
It is the same as "We ARE pay" in English. Of course, this is grammatically wrong. You can use either "We pay" or "We are paying".
Because standard German only has the simple present: "We pay."
"We ARE payING" can't be translated as such, unless by saying "Wir bezahlen gerade" ("we [are] pay[ing] just now" - you might say that e.g. to somebody who interrupts you), or by using a non-standard dialect you shouldn't really learn on Duolingo :) (but which, in case you know Dutch, is perfectly fine there - "wij zijn aan het betalen")