Welcome the the Community and thanks for taking the time to post here.
I'm really sorry to hear you are having an issue adding your device for use with the Sky apps.
The TV Anywhere and Sky platforms are separate and therefore not synced so any changes made to one, also need to be made to the other.
You can manage the devices you have registered for use with the Sky platform by going to anywhere.virginmedia.com > My Profile > Sky Movies & Sky Sports.
Go here and delete one of the devices you already have registered. This will create a free slot. Once you've done this, try steaming from the device you want to add and this will automatically register it.
Keep me posted on how you get on.
Kath_F Forum Team
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