I'm also having this problem. My YouView box works perfectly otherwise (I can watch most channels, and also box sets/catch-up TV etc) but am unable to watch BT Sport. I've tried the procedures suggested in the help pages.
Welcome to the forum and I am sorry to hear you are experiencing this problem as well. Unfortunately this isn't something that other members on the community will be able to help with so I will need to get your details.
Can you please drop me an email? If you click on my username >>SeanD<< you will find the 'mods contact link' under the 'About me' section of my profile.
Thanks for sending over your details. I ried calling you today, sorry I missed you. The reason you're having this problem is that you don't have a BT TV subscription so will need to sign up for one of our TV packs. You can find details at BT TV.