Hyde here from Shopify.
You say you've "just" linked your Namecheap domain, does that mean you did it within the past 24 hours or so? As it can take up to 48 hours for a domain to fully propagate!
Also, did you ensure to change your MX records to point to Shopify? For help with this, you'll want to reach out to Namecheap directly.
Let me know how you're getting on!
All the best, Hyde.