I do think that you are having working Internet connection so here just some additional test. Please execute:
If above sucessful please refer to this post: Dirty Pipe kernel vulnterability - #2 by Quarnero and rename Linux Kernel to: linux-image-5.10.0-15-amd64 (current one) like this:
sudo apt update
sudo apt install linux-image-amd64
sudo apt install linux-image-5.10.0-15-amd64 −− probably not needed.
sudo reboot