I don't understand why, for "Non sono americani" it accepts both "Are they not American" AND "Are they not Americans", but for this question it will not accept "We are professionals". It only accepts "We are professional". I get that one way it's a noun and the other way it's an adjective, but if it works as a translation in the one case, shouldn't it also work in the other? I'm losing heart. :)
Americani can be both the adjective used for a group of people with the blue passports (American people) or the noun denoting citizenship (Americans). Professional is different, in English it can be both an adjective and a noun, but in Italian professionista is the noun and professionale is the adjective.