Unlinking your bank account in QuickBooks Online (QBO) is just a breeze. You've got me here to help you succeed, Tom208.
May I know your financial institution? This way I can check on our end if there's a reported case of the error message you've encountered. To better isolate this issue, let's try some basic troubleshooting tests to figure this out. Open a private browser and try unlinking your bank account. The private mode prevents the cache from being retained while the website is operational, which may cause problems. Here is a list of shortcut keys you can use to access one:
- Ctrl+ Shift + N for Google Chrome

- Ctrl + Shift + P for Mozilla Firefox
and Microsoft Edge 
- Command + Shift + N for Safari

Before unlinking the bank account, ensure to add and match all the pending transactions on the Banking page. Know that disconnecting the automatic online banking updates will delete any transactions in the For Review tab. Once done, let's go ahead and unlink the account:
- Go to the Bookkeeping menu in the left navigation bar.
- Select Transactions.
- From the Bank transactions tab, click on the dropdown arrow beside Checking and choose the bank account you'll want to unlink.

- Click on the Edit icon.

- Select Edit account info and tick the Disconnect this account on save checkbox.

- Then click Save and close.
To give you more details on what will happen if you disconnect your bank account in QBO, please see this link: Disconnect accounts connected to online banking in QuickBooks Online.
If this works, go back to the regular browser and clear the cache to refresh the background process of your browser settings. But, if the same thing happens, use another supported browser.
I'm also adding here some resources that can help you in reconciling your accounts:
Feel free to get back to me here in the Community space if you need further assistance with this matter. You can also post here again if you have any additional banking-related concerns. It'll be my pleasure to cater to them again. Stay safe, and have a nice day!