Thanks for your post, however, whenever posting, please ensure to provide as much information of the issue as well as the underlying environment e.g. Linux distribution/release/update, kernel version, physical or virtual, hardware vendor/model, virtualisation solution/version, relevant package version, relevant component/configuration related information, etc.

With specific details, readers will be more inclined and able to offer specific responses, rather than generic suggestions.

That said, for security/privacy reasons, ensure not include any organisation/network/system/user, etc. specific information here to this public forum.

With regard to your issue, on a system utilising the default installed system configuration, no specific configuration is required to allow one user to su(1) to another. For regular users, as long as user1 has the correct credentials of user2, then switching to or running commands as user2 should simply succeed.

[user1@host ~]$ su - oracle
Password:
Last login: Thu Aug 14 13:10:50 AEST 2025 on pts/8
[oracle@host ~]$

What, if any, error or issue has prompted you to pose your question?

Depending on your distribution, review files /var/log/secure, /var/log/messages, etc. for any authentication related errors.