Grammar isn’t as scary as you think it is. You can probably learn enough on your own, or use a free program like grammarly, to help you enough to get by until the novel is popular enough to afford paying an editor. Editing is a lot of work to do for free. Even your promise of payment relies on your book actually being good enough to make money on here.
If there is a specific thing you need help with maybe ask for help on the forums. People might be willing to help you fix a paragraph or two at a time. They also might be willing to explain a concept like, “When do I use a semi colon?” However, I doubt you will get high quality editing for a long book for free. Remember, a lot of people on here either can make money elsewhere or have their own ideas to write.
Sorry for the bad news, but I hope this is helpful.