I don't see anything wrong with it. Did someone say it was wrong? What was the context? It could be wrong if you meant to say "She wasn't very fast at running." It is grammatically correct. It can make sense. In order for it to be correct, though, it needs to follow other sentences that "set the scene." "She was slow at everything. She wasn't fast at anything. She wasn't fast at swimming, she wasn't fast at walking, she wasn't even fast at running." The word "even" means that there were other things she was not fast at. It also means that it is surprising--you would expect her to be fast at running.

It’s not technically wrong and Dan has constructed a use that makes sense. But it’s probably an incorrect or awkward way to express what you mean.

I agree with Dan, I don't think there is anything wrong with this sentence.