Why do I keep having to 'sign-in'? It won't let me purchase.

To get back to a one-time-purchase Office and stop the sign-in loop, follow these steps in order:

1. Confirm that Office 2019 is (or was) really a one-time purchase on the account
   • Go to Subscriptions and sign in with each Microsoft account that might have been used.
   • If Office 2019 (or another one-time Office license) does not appear under that account, the email being used is not associated with that license.
2. Turn off any Microsoft 365 subscription offers
   If a Microsoft 365 trial or subscription is attached to the account and is confusing activation, turn off recurring billing so it will not keep trying to use 365:
   • Sign in to Subscriptions.
   • For any Microsoft 365 subscription shown, select Turn off recurring billing.
3. Buy a new one-time Office license (if needed)
   If there is no existing Office 2019/one-time license on any account, a new one-time purchase is required:
   • Purchase a one-time Office product (for example, Office 2021 one-time purchase) from the official Office site or store.
   • After purchase completes, do not choose Microsoft 365; choose the one-time Office product.
4. Install Office and switch the license to the one-time purchase
   After the one-time purchase is associated with the correct Microsoft account:
   • On the Mac, sign out of all Office apps.
   • Open Word, Excel, or PowerPoint. When prompted to sign in, use the same Microsoft account that shows the one-time Office license under Subscriptions.
   • If the app still offers Microsoft 365, use the in-app option to switch your license to the one-time purchase associated with that account.
5. If Office keeps asking to sign in on Mac
   If there are repeated sign-in prompts or “authentication expired” type behavior on Mac, there may be a local keychain/credential issue. Follow the steps in the keychain-reset article referenced in the context to clear Office’s stored credentials and then sign in again with the account that owns the one-time license.

If the purchase flow itself keeps looping sign-in in the browser, try:

• Ensuring cookies are enabled and not blocked.
• Using a different browser or a private/incognito window.
• Verifying that the Microsoft account profile (name, address, phone, etc.) is fully filled out before attempting the purchase again.

References:

• Switch between Microsoft 365 subscriptions
• You can't complete a purchase in the Office 365 portal
• Unlicensed Product and activation errors in Office
• What to try if you can't install or activate Office for Mac