Cash back is not showing

Same here. It only shows an option to setup Apple Cash.

Apple Cash shouldn’t be required to apply cashback to balance: Get unlimited Daily Cash with Apple Card - Apple Support

BTW, don’t expect any help from the Apple forum. When you Google for questions about Apple products it is easy to find other people with same issue in the Apple forums, but I never saw a useful answer. Usually people will repeat the Apple docs, which of course you already tried and doesn’t work, or will say, it works on my phone.

Like my answer now :)