If you receive a cloudflare - error 1001 when trying to access a subdomain on your app, this means that you have not added the subdomain as one of your app’s domains.
Because of the way that App Platform manages DNS records, you cannot manually adds DNS records to direct subdomains to your app, such as creating a CNAME record in DigitalOcean DNS to point at www.example.com. Instead, you need to explicitly add the subdomain in your app’s settings and ensure that your domain’s DNS is configured using either option 1 or option 2 in that doc.