Sub-Account AUTHENTICATOR Registration Issue

After registering the AUTHENTICATOR with this account, I tried to register the AUTHENTICATOR with the sub-account, but the following page occurred.


An error occurred and your request couldn’t be completed.

Is it impossible to register a secondary account AUTHENTICATOR?

Blizzard said that up to three accounts can be used in individual names, so I created a sub-account and even purchased a game, but I’m worried that the account is not secure. Please tell me how!