Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Log into your Microsoft account on this link and check the expiry date on your Microsoft 365 subscription.
https://account.microsoft.com/services/
If the expiry date is listed correctly for your subscription payment and, you can create and save new Office files and edit existing files, then that notification is appearing in error and there is nothing you need to do, many people are reporting that notification is appearing in the Office apps, it is a problem Microsoft needs to resolve with an update to the Office apps.