I am facing this issue in WOTLK classic.
I bought a Parrot Cage(Cockatiel) and Parrot Cage(Senegal) from a vendor. But when I try to use it, it says “Cannot Add Companion to your account”.
After some back and forth with in-game support, it was suggested that I keep trying to add the pets as the other thing they tried (deleting the cages and sending them via mail) didn’t work. Since four months have passed since, I reopened the ticket and was asked to file a bug report.
Thank you.