The error "no valid sudoers found" appears when you have modified the permissions of an important folders. You have added a user and know the system is saying that the "sudoers" are not valid anymore. This is a very serious issue.
Example of error made :
sudo chown -R username /etc
If you try the following :
sudo apt get update
You get :
no valid sudoers sources found, quitting
Solution : you can try to change the chmod of the sudoers
chmod 0440 /etc/sudoers
If it doesn't work, do it in the rescue mode.
You can try to modify the sudoers access :
Modify it like that :
username ALL=(ALL) NOPASSWD: ALL
If it still doesn't work, you will have to start from scracth with your entire server.
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