Unable to associate since your App Store purchase has already been associated to a different Microsoft Account.
The subscription you purchased from Apple is already associated with a Microsoft Account.
Subscriptions are associated with the Microsoft Account that you used to log in to your Office App when you purchased the subscription.
In most cases, this issue occurs when you purchase Microsoft 365 with a personal Microsoft Account, but then sign in with a different personal or work Microsoft Account. To troubleshoot, try the steps below:
Sign out of the Office app.
Sign-in with the Microsoft Account that you believe you used at the time of purchase.
Try the steps in this article: Restore previous purchase for Office for Mac.
If the problem persists, please contact Microsoft Support.