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 used to be annoyed by this. I’m running GrapheneOS so I denied Google Photos access to the network but it didn’t stop it from asking every few times I started the app. Sometimes it even ignored my response and tried to connect to the network anyway.
I just uninstalled it and installed Fossify Gallery instead from F-droid. Haven’t looked back since.