That is not a forwarding issue but a certificate issue. Cloudflare does not support a host on such a deep level with their free universal certificates. You will need ot purchase a $10/month dedicated certificate from Cloudflare if you want to support that host.
Yes it’s a subdomain. I want to redirect the www.my.trapo.asia to my.trapo.asia. If you’re going to www.my.trapo.asia you will that it says that the page doesn’t exist.
My guess would be that you don’t have a DNS record for “www.my” (or whatever you need when there are 2 parts to the subdomain - not something I’ve done before) and that’s why you’re getting the error.