Probably, the QuickBooks account you want to link to your real bank account is already connected to a different one. Hence, you're prompted with the error.
Here's how to review your QuickBooks bank account:
Select Accounting on the left menu.
Go to the Chart of Accounts tab and look for the bank account. Note: Accounts connected to Banking show back & forth arrows
Click the drop-down arrow next to Account history and select Edit.
If this account is connected to the wrong bank account, you can mark the Disconnect this account in save box.