In order to get the $400 Facebook ad credit, you would've needed to have followed the following process:
User signs up for Shopify trial on the offer Landing Page (shopify.com/facebook-credit or shopify.ca/facebook-credit)
User receives onboarding emails with reminding them about the offer, while concurrently building their store
User picks a Shopify plan, and is charged for their first account fee
User receives an email from Shopify with a unique ad credit URL
User clicks on their unique ad credit URL in the email, which drives to the FB landing page for ad credit redemption
User follows steps on FB landing page to redeem their credit. As part of this process, FB will confirm that the user meets FB’s eligibility criteria for ad credits.
Please ensure you review these steps to know what part of the process you still need to fulfill in order to get your credits. Feel free to send over any other clarification or questions that you may have.
How has your journey with us been so far? I have also attached this great blog post about marketing that you may find useful as well: How to Advertise on Facebook.