Hi @Direx welcome to the forum and thanks for posting. I've moved your post to create your own thread as the previous thread was already resolved.
Sorry you are having a problem casting BT Sport. We will be happy to check your Sport package for you. I've sent you a Private message explaining how you can contact the forum moderation team.
Check here: https://community.bt.com/t5/notes/privatenotespage
Yes. On an 8gb plan
Then the message is correct, you can not cast on the free BT Mobile sports app, It's mobile device only.
If you wish to cast then you need the standalone app (Only available to BT broadband customers) which costs £9 per month unless on a promotional plan, the standalone app does support other devices such as Apple TV and Xbox one consoles, Chrome Cast and others, it also gives you access to the online player at www.btsport.com
You did state you have BT broadband, which means you can order the standalone app, see what deals are available.
The terms are on the page you ordered it from which is www.bt.com/sportonthego lists the following highlighted in bold :
Free BT Sport on BT Mobile 6GB, 20GB, 15GB or 30GB: