It sounds like you have a server edition of Confluence (you've downloaded and installed it on your machines yourself), rather than a cloud subscription.
That's fine, but you've now gone into your account and tried to add a cloud addon to a server Confluence. Which won't work. Note that I say "your account" - your Atlassian account might not be the one that your organisation signed up for Cloud with.
Your best bet is to log into your Confluence and go to Admin -> Find new add-ons and search for Questions in there - it will take you to the right place.
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