"They do not make mistakes."
Translation:Eles não erram.
The reason that the exact translation of 'make mistakes' (fazem errors) does not work, I'm assuming, is simply that it is not a collocation in Portuguese. This means that it is not a recognized combination of words. It's the same reason why we can say 'heavy rain: in English but not 'strong rain'.
This is easily the most difficult confusing sentence ive encountered. This is the type of sentence that needs a lot of explanation so we understand why the sentence is like this. From the help given the best i could come up with was "eles nao fazem compreende mal" not even close..