Unfortunately "Knock off" has a whole range of meanings and without more context it is not possible to be sure. Merriam Webster gives some of them: https://www.merriam-webster.com/dictionary/knockoff Hillary famously said it after climbing Mount Everest. I suppose that fake, not genuine seems most likely here.

The term "knockoff" refers to a replica (an imitation) of something real. It implies that it is of lesser quality. You "spot" a knockoff if you distinguish it from the real thing. E.g., a fake Rolex vs a real Rolex. So here, in this instance, it refers to the difficulty in distinguishing real love from something that is superficial. In Sex and the City (and Hollywood in general) there are many examples of love that are temporary and superficial. They are "knockoffs" of real love, which lasts. If this answers your question, please vote me up.

I don't know the movie, but in this case I believe that the speaker is looking for a serious relationship. The knockoff then refers to someone who is not serious. The knockoff is just looking for casual sex and does not want to bind him/herself