
Subscriptions in MS refer to services such as Office 365 and Xbox that you have to renew. If you bought a retail copy of Office 2019 then it won't show up as you didn't buy the service, and hence need the subscription. Your copy of Office is good for life and is not tied to your MS account.
If you were to buy an Office 365 subscription then you would need to renew yearly and it would then show up under subscriptions.