I too ran into the same issue..and thanks to this blog post I solved it :)

Here's how to do it, I hope somebody will find this useful..

In ubuntu recovery mode, enter the following commands...

mount -o remount,rw /

mount --all

chown root:root /usr/bin/sudo

chmod 4755 /usr/bin/sudo

restart

That's it..it worked for me...

Source Here

pkexec chmod 4755 /usr/bin/sudo
pkexec 4755 /usr/bin/sudo

this helped here

That's in fact a pretty serious error. What was the actually command you ran? Did you recursively change the permissions on all files? If you have root login enabled, you can potentially recover sudo Login via the console in the DigitalOcean control panel and run:

chown root:root /usr/bin/sudo
chmod 4755 /usr/bin/sudo

You may still continue to run into problems with permissions on other files as well. For instance, you most likely can't access the droplet via Putty due to the permissions on you SSH files being incorrect incorrect. If you have a recent backup, your best option is to restore from it.

What I ran is chown command that gives my another user an all access to root folder. yeah i think i need to reinstall the OS

how do I switch to ubuntu recovery mode via putty?

TK! It helped me!