Multi-variation listings are not allowed in all categories. Last time I checked, it was not allowed for books. There is a list ebay for what is allowed and what isn't, but I don't think it is up to date. The best way to know for sure it to just try to create a multi-variation listing. I know it works for clothing, so do a practice try using different colors of say for instance t-shirts.
For items that DO allow multi-variation listing, you just need to start a brand new listing. Don't do "sell similar" of something else. Start a completely new listing. You will get the prompt while you are entering the other details, it is a radial button right on the listing, it asks you if this is a multi-variation listing. You usually just ignore this prompt in the past, since you weren't doing it before. Now look for it when you are creating your listing.
Good luck.
