What your server uses (Windows, Ubuntu, Debian, etc.)? The exact error message printed in your server logs?

TeamSpeak 3 offers a free version that allows users to experience most of its features. There are limitations, such as a maximum of 8 licensed slots (meaning you can have up to 8 users connected with a license), but for personal use or small groups, this often suffices.

How to accept the server license agreement (Server >= 3.1.0)