Unable to switch from one user to another user using su on RHEL server

Issue

• Users are unable to switch to another user account using the su command.
• Direct SSH to the target user works, but switching internally via su fails.
• Error received:

[testuser@testserver ~]$ su - suuser2
Password: 
su: Authentication failure

• The su command works for the root user, but not for non-privileged users, confirming that the command itself is functional.