One problem new users sometimes have after installing ThinLinc is not knowing which username and password to log in with. In short, ThinLinc uses whatever authentication mechanism (or mechanisms) the server OS is configured with. See:
How do I create a ThinLinc user?
In other words, you should log into ThinLinc using the same username and password as you would if you were sitting in front of the server; just make sure you’re not already logged in as that user on the same machine first:
Why do I get stuck on a splash screen when starting a ThinLinc session?
Note that during ThinLinc installation, you will be asked to provide an administrative password. This is the password for the admin user on the web-based administration interface, and cannot be used for creating a session. To change this password, use the tool tl-gen-auth:
https://www.cendio.com/resources/docs/tag/man/tl-gen-auth.8.html