You have a genuine mystery issue here. You already verified that users “ubuntu” and “nvidia” are in group “adm” within “/etc/groups”. This probably won’t reveal anything interesting, but as users “nvidia” and “ubuntu”, if you run the command “groups”, does this too list group “adm” for them?
Your ext4 filesystem type is good to go, and the actual error message implies it isn’t a failure of the sudo command itself not being suid root. This tends to imply that “/etc/sudoers” or something in “/etc/sudoers.d/*” is at fault, and this may be something external software did interfere with during a software install.
Normally nothing is in “/etc/sudoers.d/”, at least not by default. If something is there (other than a “README” file), then this is non-default. Is there anything in directory “/etc/sudoers.d/” other than a “README”? If so, this is of interest.
If “/etc/sudoers.d/” is empty (other than “REAMDE”), then we need to know the content of “/etc/sudoers” file.
For any of the files you find in “/etc/sudoers.d/”, and for the actual “/etc/sudoers” file, if possible, save a copy of each with a “.txt” file name extension, and upload here. These files do not contain passwsords so there isn’t any issue with posting them.
In the case that there are files (other than “README”) in “/etc/sudoers.d/”, then I suspect renaming them would be a fix, but that you won’t be able to rename due to the sudo problem. On the other hand, if we know of some new user name being added, then maybe there is a different login which will work. If no other path is available to edit or correct one of these, then there is a way to clone and restore after editing on a host PC (but this is very time consuming).