Many have this since updating to 15.0
In short, your system cannot be upgraded anymore. Restoring etc.. will not work either.
There is a way to upgrade but for this you need a second macOS device running 15.0 or newer. It requires to put your system in DFU mode and then upgrade via the second device. If you do not have a second device, you need to bring it in for a service (which is not free btw). :-(
In short.. Apple f*cked this really up.
Here is how to do this (DFU+2nd device):