Hi lost123,
I'm here to help you fix the error you encountered so you can continue working on your bank transactions.
It's possible that both the parent account and the sub-account are connected to online banking. Thus, the system prompts you with that error message.
To fix this, let's ensure that either of the accounts is connected to online banking.
Moreover, I've added this article as your guide in categorizing transactions: Categorize online bank transactions in QuickBooks Online.
Keep me posted whenever you have banking concerns.
