I'd like to share some information about this process, hfulford14.
You will get this error when both accounts are connected to online banking. Since you're unable to connect the sub-account to the main account, you can either do the following:
I've got an article here that discuss more about the process: About bank or credit card sub-account setup.
Note: Disconnecting an account will remove all of the downloaded transactions in the For Review section. Please make sure to categorize the transactions before following the steps below, so the data will be added in QuickBooks Online.
Here's how you can disconnect an account:
I'm adding a sample screenshot for a visual guide below:
Once done, you can edit the sub-account in the Chart of Account and select the main account from the Is sub-account drop-down.
You can use the following articles for reference:
Please reach out to me if you have any other questions and I'll get back to you the soonest. Have a good day!