Hi @Joannemeadows and welcome to the forum.
Did you move from Sky Q or any other Sky product to Stream as if so all your Marketplace subscriptions are cancelled so you have to re-register (see the THIS link). Even if you didn't transfer you still need to register for Disney + (even though it is free with Sky)
You can register or re-registered (as appropriate) by doing the following: Use the voice command "My Sky" and then select the following: your products / (enter your PIN) / App Subscriptions / Current / and then Discovery + Monthly. Hopefully it will say £0 per month whilst you have a Sky TV subscription. If so select Activate subscription.
Also, see the following link for more information about registering for Discovery +:
https://www.sky.com/help/articles/sky-marketplace-subscribe-to-discovery-plus
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