Offers linked to device purchase must be redeemed within a specific period (often within 30 or 60 days - or by a specific date), otherwise they are lost.
You can contact the Support Team using the Support link at top-right or bottom-left of this page. Having evidence of the offer to which you think you are entitled, such as a screenshot, may help.
Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044