How to turn on "sudo" in OSX 10.12 Sierra

Open up Terminal and enable root with:

dsenableroot -u <your_admin_username> -p <your_password> -r <root_password> #choose some root password here 

If the sudoer's is damaged, you still won't be able to sudo - but you will be able to su and repair the permissions accordingly:

su #when asked afterwards enter the above <root_password>
chown root:wheel /etc/sudoers
chmod 440 /etc/sudoers
chmod -a "admin allow read,write,append,readattr,writeattr,readextattr,writeextatt‌​r,readsecurity" /etc/sudoers
exit

Back in your admin account disable root:

dsenableroot -d -u <your_admin_username> -p <your_password>

In case the above won't help, re-enable root account doing the same as above, logut of your account, login as root (yup, really):

Open terminal and type chmod g+x /, close the term, logout and tada, sudo is working again :) This problem happens mostly when you mount your / through linux in rw mode (for hfsplus in r/w mode use fuse and bindfs)

BTW the group admin ACL doesn't seem to harm the sudoers file!