Cannot purchase Realms Server subscription on Xbox - "You have the maximum number of active subscriptions"

From the main UI I go to settings>Subscriptions and try to buy a £3.29 option for a private Realms server but get the error "You have the maximum number of active subscriptions on this platform and cannot create a new one at the moment"

I have no Minecraft subscriptions at all and this is my first attempt at purchasing one.

All I have is GamePass Ultimate.