You can know if a user has a password with the command
sudo cat /etc/shadow.
If the second term is a string like "jbpLYuW97V0pDwukqaagcRu9yLWYp/O7bHr5baf6DLgj5" (much longer) so the account has a password. If not, not
So I looked, Plex user doesn't have password.
You can login with Plex user by these commands :
su plex or
sudo su plex (If you don't have sudo rights, it will ask you a password even if there is no password)
And you can use
sudo -v to know if the account has some sudo rights, if you have some, no message will be displayed to the console. But if the account doesn't have rights, you will see :
Sorry, user plex may not run sudo on YourHostName.
So I think the plex account should not help you
Anyway, did you set a password for the root account ?