When I try to connect custom domains with some extensions with .cf, .tk, .ml (or we can say freenom domains)to test my apps, it is giving am 800000 error or something like that. But I successfully connected .com and .org domains many times in my personal account. It is working for that. So, it said me to contact support.
Have you followed the tutorial here?:
Have you tried to connect it via API or using Cloudflare dashboard?
Is the domain name correctly pointed to Cloudflare nameservers?
I am afraid I am not familiar with kind of an error like that one