to say what Amy said a bit differently, the ability to set user privileges is not part of the freely usable “team edition” builds of Mattermost. This has previously been discussed for example in Adjust member's authority on channel
For technical reasons the code to handle set permissions that have already been set are part of the open source code (since these restrictions are stored in the database). In previous version you could patch the server code to show the ui to change these permissions (it was a simple “has license” check somewhere, will not share the patch here out of respect for the Mattermost developers), but you could also temporarily run the enterprise edition with an evaluation key, set permissions and then go back to the free team edition.