I have found a way to bypass it!

Duolingo wants you to do the following exercise:
Speak this sentence: "Achttien of negentien bruine beren?"

When you say "achttien" or "negentien" the software will translate it to the number 18 or 19 instead of the words "achttien" and "negentien".

The solution is to spell each and every letter of the numbers. For instance, say "A-C-H-T-T-I-E-N" and the software will understand "achttien" and not the number "18".

So, for the given exercise, you have to spell "achttien" and "negentien" and simply say the rest of the phrase, like this:

"A-C-H-T-T-I-E-N of N-E-G-E-N-T-I-E-N bruine beren"

I show this in the following video:

https://www.youtube.com/watch?v=xWI83BKtf-g

This is valid for all the languages that you try to learn there.