If you see the “Could Not Fetch” error on Google Search Console, try writing a short blog post and publishing (you can delete it later, it’s only temporary). This worked for us, and the reason I’m thinking it did is because Shopify includes a link to your News blog by default in the sitemap but somehow Google can’t read the sitemap if there aren’t any posts and the News blog is an empty page. OR it could be because the new blog post changes the sitemap so it “forces” Google to re-read it, so this still might work for you because you already have blog posts. That’s my guess.
Not very scientific but if you search the Shopify forums this “solution” has worked for others in the past too. Unfortunately it doesn’t seem like there’s an official Shopify approved solution or any indication from them why exactly the error might happen. Hopefully just adding a custom domain solves it for you though!