I run into this issue when upgrading my My raspberry pi 4B personal server setup from Debian 11 bullseye to Debian 12 Bookworm.

The solution was found here. In short:

sudo umount /boot 
sudo sed -i "s:/boot:/boot/firmware:" /etc/fstab 
sudo mkdir /boot/firmware 
sudo mount /boot/firmware

The cause was that the mount location has been updated and if you are upgrading (which Raspberry heavily dis-recommends btw) then it’ll end up in the wrong location.