Hi there, Treehouse.

The reason why we're unable to add the credit card as a sub-account is because transactions are already downloaded in the parent account if it's already connected. 

When it’s time to reconcile, you only need to reconcile the parent account because all transactions in the subaccounts roll up into it.

Let me share another article for additional details: Connect bank and credit card accounts to QuickBooks Online.

I'll be around if you need anything else.