I'd like to ensure that you're able to connect y our bank with QuickBooks.
To begin with, I'd like to ask what specific financial institution are you trying to connect with? This can help me check if there are any issues related to this matter.
When you're unable to authenticate the account it might be that you've entered an incorrect value, exceeded the allowable attempts, or the code sent for the verification is incorrect or expired. I'd recommend contacting your financial institution to verify if there's an ongoing issue that leads to this problem.
I've added a reference you can use in the future when you're ready to assess your business financials: Reconcile an account in QuickBooks Online. This also contains sub-topics on how to get a report of the reconciled account and how to adjust reconciliation.
Please don't hesitate to tag my name in the comment section below. I'm here to assist you further. Take care.