May I know what is the specific error message you're getting when connecting your bank to QuickBooks Online? This way, I can check if there's an ongoing investigation reported for that issue.
For now, let's make sure to sign in to your bank's website and check if there are no maintenance updates. Otherwise, check the error codes in this article: Troubleshoot issues with Bank Feeds.
You can also import your bank transactions manually by following the instructions in this article: Manually upload transactions into QuickBooks Online. Note: Once your account is connected to online banking, QuickBooks automatically downloads the latest transactions. In case you see some duplicate transactions, you can undo them by following these steps:
Go to Transactions > Banking.
In the For review tab, put a check mark on the duplicate transactions.
Click the Exclude button.
I've got this sample screenshot for a visual guide:
After excluding these transactions, go to the Excluded page and delete them.