Hi @TiborH and welcome to the forum. As you don't mention what is happening when you try to activate your Stream I will post a fairly generic answer which hopefully wil get you up and running 🤞
Are you signed into your My Sky account at https://www.sky.com/mysky If not sign in before entering your activation code. You must also be using the same email that is registered to your account.
Once logged in go to https://www.sky.com/productsettings/streaming-tv/activate This is where you can enter your activation code that appeared on your screen during setup.
The following link is to a setup help guide:
https://www.sky.com/help/articles/setting-up-sky-glass-help
If all else fails call Sky for some additional troubleshooting.
Good luck.
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