
Even though you're part of a monthly paid Microsoft 365 family plan, you need to make a purchase because your Microsoft Entra ID tenant has been inactive. While the family plan covers your subscription costs, it doesn't count as activity for your Entra ID tenant.
Think of a tenant as the specific space that separates your account from other customers. If a tenant isn't used for more than 200 days, Microsoft requires a purchase to keep it going. This policy just helps make sure resources aren't tied up by inactive accounts.
You received the notification because your associated tenant is the one that has been inactive. The main account holder might not have gotten a notice if their tenant is active or if their notification settings are different. It's a good idea to check those settings to ensure the right people are informed about policies like this.