As it is a question, the strict form of this sentence in French should be "cela faisait-il professionnel ?". But, particularly in speech, the French often use affirmative sentences and add a question mark at the end (raising their voice with the last syllable).
"Faire professionnel" means "to look professional" or "to seem..." or "to sound..." depending on the context, that we don't have here.
You can imagine a comment about someone wearing jeans instead of a suit, to go to the office, and someone else disapproving of it.
Is that why my reply, "Was that professional?" was counted wrong while a correct answer was "That was professional?" ...just because the French is stated in the positive? In English, it sounds much better as a question although I know that we, too, sometimes use a positive to ask a question.
The same thing happens with some words in English too, when the root word has another syllable or more added to it.
For example parent is pronounced [ˈpeə.rənt], but the adjective parental is pronounced [pəˈrent(ə)l], with the first syllable being softer in the adjective than in the noun.
This tense l“imparfait is introduced in the Tips and Notes at the beginning of this series of exercises. It is good practice to access Duolingo on a PC before starting any section on the tree so you can get an introduction to the subject covered in those exercises. For this section, here are the notes you missed: Click this link then scroll down to read Tips and Notes
Oh dear! You are right; the link doesn't work anymore.
Here's how to find the Tips and Notes when you are on the Duolingo Home page. If you hover your mouse over the section of the tree you are working on, (in this case I hovered over the section for Past Imperfect) you will see a pop up that looks like this and I have indicated where the tips are in that image.