s io="whoami" o io:"rq" u io r x u 0 w x

Above command gave username as "irisusr" which was not the user added to /etc/sudoers file.

This was the issue. (User logged in the VM was "ubuntu" and I tried adding this user to the file)

When "irisusr" was added in /etc/sudoers file, sudo commands seems to be working.

Additionally, I tried the command

w $ZF(-100, "/SHELL", "sudo whoami")

which was not correct, it should have been

w $ZF(-100, "/SHELL", "sudo","whoami")

sudo commands are working now.