Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Did Costco give you a product key code? If so, you need to go to office.com/setup to redeem it, and then associate it with your Microsoft account.
This is consistent with Costco's instruction too: https://www.costco.com/wcsstore/CostcoUSBCCatalogAssetStore/Attachment/Activation-Office365-160614.pdf
If you don't receive it, please reach out to Costco support to verify that your purchase should include the 1-year subscription.