When i try domain to my own domain it says Hostname not permitted.
how can i fix it? Am I getting this error because I’m using a trial version?
Possibly. Are you referring to the hosted management from Discourse (Discourse pricing | Discourse - Civilized Discussion)?
I’m not sure you can specify your custom domain name while on trial, even if you’re trialing the standard hosting (or above). 
Is that the actual reason or is there any other technical problem related to this?
I am facing the same challenge and i am also on trial but subscribing for the standard plan 100$/month