Hi there, @Irrori. Let me give you insights on how QuickBooks Online works with bank feeds

Are you trying to create sub-accounts to the parent account? The error message you're getting comes up when the parent account is connected to online banking. You'll have to disconnect it first before making any changes.

Here's how:

1. Go to the Banking menu.
2. Select Banking at the top.
3. Choose the account you want to change, and then click the Edit icon.
4. Click Edit account info.
5. In the Account window, check the box next to the Disconnect this account on save field.  
6. Click Save and Close.

You can also refer to these articles. The steps are laid out on how to add sub-accounts:

• Create a new account or sub-account in QuickBooks
• About bank or credit card sub-account setup

Once done adding the sub-accounts, reconnect its parent account.

Additionally there are two ways how your banks send the downloaded transactions to the program:

1. Transactions download to one account.
2. Transactions download to the individual accounts.

You'll need to connect the parent account only if the transactions download to one account. Otherwise, connect the sub-accounts if the transactions download to individual accounts. If you try to connect to both, you will receive a message that the account has already been connected to QuickBooks Online.

When you reconcile, you only need to reconcile the parent account because all transactions in the sub-accounts roll up into it. 

I'm always here if you need any more help working with bank feeds. Have a great day.