Linux version 5.11.0-7614-generic (buildd@lgw01-amd64-025) (gcc (Ubuntu 10.2.0-13ubuntu1)
Kernel was built with gcc 10.
gcc version 9.3.0 (Ubuntu 9.3.0-18ubuntu1)
But you (now) have v. 9.
Guess you set that?
I told you to look at /proc/version.
Check if gcc and g++ v. 10 are installed. And if yes, use update-alternatives.
Purge and reinstall the driver using apt.
You just need to install cuda-toolkit, not complete cuda (which would try to install its own driver when following the nvidia guide).