"She is not fast."
早い is a い-adjective, meaning that you need to conjugate it based on what you do with it. If attatching it to a noun, then you can leave it as it is... see example below. 早い車- Fast car
Now if we attatch it to a verb, we need to change the final い to a く. An a example below. 早く走る- run quickly.
Negatives for い-adjectives are based off of ない (negative ある). Because that is a verb (of sorts), we change it first to the adverb form (早い to 早く) and stick on the ない, like in the question. 早くない (not quick)
Past tense of い-adjectives take on かった in place of い. 早い to 早かった. (was quick)
ない is also treated as an い-adjective, so past negative would be... you guessed it! 早くなかった (wasn't quick)
彼女ははやくないです not accepted? Not sure why. It says かのじょははやくないです, I thought 彼女 could be used for both 'she' and 'girlfriend'. As for 'fast', I thought 早く meant early and 速く meant fast. but on other questions every time I have answered 速く for 'fast' it has marked me as being wrong.