Sudo folder empty/broke
Reinstalling the OS will fix /private/var and may well be needed. I’d start there, and test sudo again. 

Next consider booting to single user mode and make a new admin user by removing one file only - rm /var/db/.AppleSetupDone

Then you’ll know if sudo and your admin account and system are operational or you need to erase and start over to restore files from a backup. 

Neither of the above steps will erase any files you have - just will correct system files and create a new user with clean admin permissions and home folder. That doesn't mean if your drive is failing, this won't erase everything or your drive is immune to failure - just that Apple macOS install doesn't erase any user data or settings - it's designed to upgrade everything in place and fix things. You have to jump through lots of hoops with warnings to erase and then install. A normal install is designed to be safe to your data.