It seems that Brand A was not created by you, while Brand B was created by someone else. MailBluster allows the same AWS account to be connected to multiple brands, but all those brands must be under the same MailBluster account. In your case, the ownership of these two brands is different, which is why you are unable to connect the same AWS account to both brands.
If you’d like to change brand ownership, our support team can assist with that, so both brands can exist under the same MailBluster account. After that, you’ll be able to use the same AWS account for both brands.