Hey there @fuzzyfelix! We are so thrilled to see you so excited for Horizon Worlds, and we are happy to help you get this figured out.
First things first, you'll need to make sure that you meet the requirements for using Horizon Worlds. You'll need to be 18+, and in one of the supported countries for Horizon Worlds. As long as you meet those requirements, you should be able to download it.
If you meet those requirements and you're having trouble getting it, please check that you're not connected with a VPN as that can cause some issues here.
And if you still can't get it then, you should submit a bug report on the Horizon Worlds Feedback Forum. That is going to be your best path towards a resolution here!