Bsrgvty

When I type sudo on terminal, I get:

sudo: unable to stat /etc/sudoers: Permission denied
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

More info:

ls -ld / /etc /etc/sudoers
drwxr-xr-x 24 root root 4096 Dez 21 22:44 /
drw-rw-r-x 162 root root 12288 Jan 18 15:13 /etc
-r--r----- 1 root root 746 Jan 18 14:21 /etc/sudoers

pkexec visudo:

Defaults env_reset
Defaults mail_badpass
Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# User privilege specification
root ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:
#includedir /etc/sudoers.d

Run pkexec chmod 755 /etc to fix this.

As steeldriver suggested, this error is because /etc has the wrong permissions. The permissions shown from ls -ld /etc should look like:

drwxr-xr-x

But yours look like:

drw-rw-r-x

This is preventing sudo from traversing /etc to access /etc/sudoers.

The "permission denied" error message just trying to retrieve metadata on (i.e., to stat) a file nearly always means the file is missing or directories leading to it cannot be traversed, and when you ran ls on the file, it was shown to exist.

I have tested this (on a virtual machine) by breaking permissions on /etc with chmod ug-x, and was able to produce your exact error message. (You shouldn't run chmod with ug-x on it -- that's how I deliberately broke the permissions, not how to fix them.)

To fix the permissions on /etc, give the user and group owners executable permissions. The absence of those permissions is the cause of this problem. In addition, by default /etc doesn't need the group owner to be given write permissions, so unless you know you want this, I recommend changing that back too.

This command will reset /etc to the default permissions:

pkexec chmod 755 /etc

That should work for you since you were able to run other commands as root with pkexec. Other users who find this by searching will usually be able to fix it the same way, but for systems without pkexec (or where pkexec will not work) it may be necessary to boot into recovery mode or from a live CD/DVD/USB to fix the permissions. You shouldn't need to do that, though. Just run that one simple command, and sudo should work again.