Unable to associate Office 365 with my account

For this error, the supported checks are:

1. Close all Office apps.
2. Go to Services & subscriptions: https://account.microsoft.com/services
3. Sign in with the Microsoft account that was used for the Microsoft 365 purchase.
4. Check whether the Microsoft 365 subscription shows as active.
   • If it has expired, renew it and then restart Word or another Office app.
5. If the subscription is active, go to Payment options: https://account.microsoft.com/billing/payments
6. Check whether the payment method has an error, such as an expired or cancelled card.
   • If there is an error, update the payment method, then restart the Office app.

If the product was bought from a retail or online store, the purchase must be redeemed first so it becomes associated with the Microsoft account. If the product was bought from the Microsoft Store while signed in, the account used for that purchase is the associated account.

If someone else installed Office and their account became associated with the product instead of the purchaser’s account, support must handle that case and proof of purchase is needed.

References:

• Office error "Account Notice: We've run into a problem with your Microsoft 365 subscription"
• Why do I need an account to sign in to Microsoft 365 or Office?