I'm on a MacBook Pro M1. Are you able to add an album from your photo library, or any single photo at all, as a wallpaper? You've probably already opened the Photos app and allowed it to update everything; if not, do that first. In Wallpaper settings, I had to select individual photos, then switch back to the "shuffle" option, and then change the "Shuffle" time to something other than what it had been before (15 min), to get macOS 15 to actually display my photos instead of the default trees. I recall having had similar problems with Sonoma. The OS wasn't respecting the existing wallpaper settings. Not sure what's up with that, other than the fact that Apple really wants to be sure we all see those trees. 🙄