We did not use to drink coffee is a correct phrase.
The implication is that we drink coffee now. We didn't drink it in the past, but now we do.
You could also say "We were not used to drinking coffee", it's also a correct phrase. It means that in the past, we weren't familiar with drinking coffee, or we didn't do it often.
"We didn't use to drink coffee. Now, we drink it every morning."
"We weren't used to drinking coffee. When we drank espresso, it was a real shock!"
Not sure which one better matches up with συνηθίζω here.
There is no such expression in British English as did not use! To drink coffee (Not sure it exists in American English either). If a past tense, it is as already raised (used) is the correct word. Does Duolingo ever correct its mistakes as many mistakes I have seen still remain months after?