I've never had to manually delete kernels.
Apt autoremove automatically takes care of older kernels
Maybe try reinstalling your default stable kernel linux-image-amd64 (or whatever architecture)
Typically, no need to specify specific kernel versions like 6.12
This particular community is full of totally unhinged people who dont know the difference between privacy and anonymity. 90% of people here act like they're living in north korea and will be disappeared if their phone number leaks