First I'd like to point out that the domain registration is separate from hosting, they are not required to be in the same account. If you'd like, the domain can simply be pointed to your website with nameservers or DNS records.
The most common reasons for this to be disabled would be a Transfer Hold or Protected Registration. These are to help keep the domains safe in your account if your password is ever compromised.
A Transfer Hold is for 60 days after registration or certain changes to contact details, this automatically goes away after the 60 days.
If you have Protected Registration it would need to be canceled before moving the domain, if this was purchased but never set-up it would need to be set-up before being canceled.
Another possibility is some registries have their own unique procedures for transferring domains. Though it is hard to tell which of these apply to you, if you need help post more details about your situation.