Beginning with Patch #114, new Conquest Tickets purchased from Horizon are no longer be tradable. Old tickets obtained prior will still be tradeable up to a certain period.
In order to use your old tickets, you'll need to convert them in-game, before entering Conquest by hitting the "Enter Conquest" button. Once you convert your old tickets to non-tradeable ones, ALL tickets currently in your wallet will be converted.
If for any reason, you do not wish to convert all of your tickets, you can simply move the ones you wish to keep as tradeable to another wallet before converting the rest on your account.