We’re aware of an issue where the error BLZBNTAGT0000138F happens when trying to install WoW classic. This is already being investigated by our developers.
As a workaround, it can help to open the Mac Privacy settings and in here the App Permissions. In the App Permissions allow both the Agent and the Battle.net App full access.
If this doesn’t help, we can only ask for your patience, while this is being investigated.
I do apologize for this error being caused.