You open Office on your new device and receive a message that your offer has expired. You’re then given the options to try Office or enter a product key.
When you purchase a new device that comes with an offer to redeem a subscription or Office, there is a limited amount of time for you to redeem the offer. Your limited time starts when you activate Windows. When you launch Office, you will be shown the last date to redeem the offer. If you don't redeem by this date, the next time you launch Office you will receive the message that the offer has expired.
Your options are to try Office 365 or purchase Office. The offer is no longer active.