subscribe to microsoft window, downloaded app but need verification key

If Excel and Word are asking for a product key, the usual fix is to sign in with the Microsoft account that was used to buy the Microsoft 365 subscription.

1. Open Word or Excel.
2. When the activation prompt appears, choose Sign in instead of entering a product key.
3. Sign in with the Microsoft account linked to the subscription.
4. If the apps still do not activate, go to the Microsoft account, Services & subscriptions page and confirm the subscription appears there: Microsoft account, Services & subscriptions page

Important details:

• For Microsoft 365, a product key is usually not needed after purchase. The subscription is activated by signing in with the correct Microsoft account.
• If the product key was already redeemed before, it cannot be used again for installation or activation. In that case, use the Microsoft account associated with the purchase.
• If this is a new, unused product key card for Microsoft 365, it can be redeemed at www.office.com/setup.
• If Office came preinstalled on a new device and shows a PRODUCT NOTICE banner, activation may require starting a Microsoft 365 Family trial, signing in with the account associated with Office, or buying Office.
• If the key does not work at all, contact the seller and request a refund, because unauthorized or blocked keys are a known issue.

References:

• Using product keys with Microsoft 365
• "This product key has already been used" error in Office
• Where to enter your Office product key