Registarars often 'lock' domain names to prevent unauthorised changes being made.
This can be referred to as 'Domain Locking' or 'Client Transfer Prohibited'.
A WHOIS search on the domain may show the following message if it is locked.
Domain Status: clientTransferProhibited https://icann.org/epp#clientTransferProhibitedIf your registrar doesn't offer the option to unlock the domain, you should be able to contact them to request this. Once your domain is unlocked, it should be available to transfer.
NOTE: Be aware of the 60 day transfer lock before unlocking your domain.