I ran into the same problem. There are quite a few steps you need to do manually (i.e in the Web Console) before you can use this guide. I won't get into details, but first you need to register a domain name in Route53 (or add a hosted zone with a domain from another registrar, but that is probably more complicated). Then you will need to add a certificate. Easiest way is to use the Certificate Manager. Enter yourdomain.com and *.yourdomain.com in your certificate request, then follow the guide to verify it. After you've done this, you are ready to go with sls create_domain. It will add the vue-ssr subdomain for you.
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