It's so funny how Duolingo users have these practice-related inside jokes (Like electric shoes, the man in the bathtub, or taking the tiger out of the zoo)!
It should be. Next time you do the exercise, open the comments and tick the box 'My answer should be accepted'. Also, it might be interesting to say 'We are not popular' and see if it accepts that. If so, it just needs to be programmed to accept the apostrophised version.
The suggested answer was "We are not popular". I checked in the "report" area and the only options were wrong answer, wrong sound, or bad sentence. I'll look again next time.
Thanks for the feedback Misha. I'm sure there's always an option 'My answer should be accepted'. I know because I have to tick it quite often. :)
I just checked on a different puzzle and there were MANY options, including "answer should be accepted". But on the one in question, there were for sure only three. No idea how we report THAT bug.
Typical Duo bug. Probably the type you find in the soup. Thanks for letting me know. Don't worry about it, and just remember not to apostrophise next time you do the exercise. Take care :)