Adding to lukemurraynz response above, If your subscription is already registered with Microsoft.Compute, but you are still encountering issues when trying to create a shared Azure Compute Gallery, here are a few other things you can try:
Make sure that you have the necessary permissions to create a shared gallery. You will need to be the owner of the subscription or have the "User Access Administrator" role in order to create a shared gallery.
Check to see if the "RBAC + share directly (Preview)" option is available in other subscriptions or resource groups within your Azure account. If it is available in some but not others, it could be due to a permissions issue.
If you are still unable to create a shared gallery, you may want to try creating a standard gallery instead. Standard galleries are not shared across subscriptions or tenants, but they may still meet your needs.
Hope this helps.
If you need further help on this, tag me in a comment.
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