Let me share some information so you'd be able to get rid of the error and connect your bank.
To start with, what is the name of the bank you're trying to connect with QuickBooks? This is for me to further check the root cause of the issue.
In the meantime, here are some reasons why you're unable to connect your bank with QuickBooks:
Your bank recently made some changes with their security feature or connection.
Your financial institution's credentials entered incorrectly.
Once completed, you can try reconnecting your bank.
Click the Banking menu.
Tap on the Add Account button.
Locate your bank and click to select it.
Copy the URL listed in the window.
Sign in using your bank login credentials.
Update all necessary information.
However, if your bank is using the old bank feed system, you can manually upload your bank transactions to your QuickBooks account. You can go to your bank's website and follow the instructions on how to download them. Here are the supported file formats below:
Comma-Separated Values (CSV)
QuickBooks Online (QBO)
Go to the Banking menu.
Choose the Banking tab.
Hit the Upload transactions button.
Click Browse, then choose the file you downloaded from your bank.
Select the account you want to upload the transactions into from the QuickBooksaccount▼ drop-down menu, then select Next.
Follow the on-screen instructions, then click Next.