Looks like that the rebase is already in progress, so you have to wait for it to complete.
You can keep an eye on the status by running rpm-ostree status. There should be a line starting with State:, which should tell you if it’s busy or not.
If it says State: idle, you can try running rpm-ostree upgrade again at that time.
Thanks for the guide. I was able to run rpm-ostree upgrade, but doesn’t prompt to upgrade after reboot.
I got a different message in Software; unable to upgrade to Fedora Linux 35:
Invalid refspec ostree://fedora/35/x86_64/silberblue
In terminal,
$ rpm-ostree upgrade --check
1 metadata, 0 content objects fetched; 592 B transferred in 1 seconds; 0 bytes content written
No updates available.
I think you may need to uninstall the rpmfusion repositories first. I suspect since they are versioned as 34, they aren’t found in the 35 repository.