Is this wrong translated?
"They don't eat fish" I wrote: 他们不吃鱼 But the system correct me this way : 她们不吃鱼 Are they not both correct?. Thanks.
I think you're correct. Unfortunately, the system will only accept a single predetermined pronoun. You either have to look in the word bank to determine which pronoun it wants, or guess and risk a wrong answer.
The first one is a group of men and/or women who do not eat fish. The second is for a group of women who do not eat fish.
她们 is actually not being used in formal Chinese. When 他们 is used, gender is not being considered.