Can't su to user other than root on CentOS

What permissions are granted in /etc/sudoers to admin?

Also, as root, you could always su - anotheruser. An extra step perhaps, but without knowing anything about your sudoers file this will work.

In case it's useful to someone else: I just ran into the same symptoms but the answer had nothing to do with sudo configuration.

Instead, it mattered which user I was trying to su to. The target user was a service pseudo-user (jenkins) which had /bin/false as its shell. The fix was to change the shell to a valid one (using chsh).

Check /var/log/security and /var/log/auth.log.

Why are you using sudo to use su? You don't need to do that unless you change your standard ACLs.

If you use sudo, then the issue might be related to you sudoer file. Verify you have it configured properly.