It appears every few times I open the app and the usual android back button doesn’t make it go away, and there is no cancel button. What you have to do is toggle the toggle, then the button will change to “do not back up” and then you can press it. It annoys me how easy this makes it to accidentally turn on backup.
I didn’t realize that I had turned it on on my prior phone. Welp, my phone absolutely died one day and would no longer power on. I was really sad about losing my photos and videos, but then when I got my new phone I was happy to find out that at some point I turned on photos cloud backups.