You can appeal to the Upper tribunal (Tax and Chancery Chamber) if you’re not happy with the verdict. You must ask the First-tier Tribunal for permission first.
You can also ask the First-tier Tribunal to set aside their decision if you think there was a problem with the tribunal’s procedure, eg you didn’t receive a notice of a hearing.
Read the guidance for details.
You can’t complain about the decision - you can only complain about the tribunal staff or the service you received.