That sounds strange, you are sure that it’s not a ticket that is already a child ticket ? 

Yes, I have just create a new ticket for them in their workspace and tried to add a child ticket but cannot do it. 

I can’t even create a child ticket and I have global admin permissions! 

Hmm, don’t think prefix should be a issue but what are you using ? 

Any business rule that (maybe) disabling. I can’t remember if you can do that,  

Still a issue contact support@freshservice.com