If I understand properly you're trying to change the pass of the root user using only an statement and pressing enter with no more confirmation of the process.
I found that your statement, runs properly if you add a "sudo su" prefix.
~$ sudo su
Considering that you're trying to change the pass of the root user, you must to know what is the root user name and to be logged properly. You can do this with the sudo su command.
Then its enough with runs the follow command replacing "root" with the current root name, and "my pass" with the desired password for this user (remember to take a note for try this method):
~$ echo "root:mypass" | chpasswd
After you can probe entering a exit command and login command using the new password.