The connected realms feature is still relatively new, so naturally some bugs and issues have popped up. One of these hiccups in the system impacts pet collectors, specifically TCG loot redemption.
At the moment, if a player on a connected realm attempts to redeem a TCG loot code from Landro Longshot in Booty Bay, it's likely they'll receive an error. I'm not sure of the technicalities behind it, but it is definitely related to realms that have recently connected with another one. Perhaps the Landro that they see isn't the "right" Landro that they need.
Players have been reporting this issue since the end of last year, and it appears to still be a problem. From what I gather, Blizzard is aware of it and are working on a fix but there's currently no ETA. Game Masters can provide limited information, but may not be able to assist with a workaround since simply giving players their TCG loot would result in duplicate items once the system has been sorted out.
If you have already redeemed your code and are simply waiting to turn it into Landro Longshot, you'll probably have to wait until developers fix this problem.
If you have NOT redeemed your TCG pet code yet, I recommend creating a character on a non-connected realm and redeeming the pet there. Since pets are account-wide, you'll still have access to the companion via Pet Journal even though it was redeemed on another server. That's the only workaround that I've come across so far, and keep in mind it only works with TCG loot that's account-wide. For non-pet/mount items, we'll just have to sit on those codes and wait.