First of all, I'd like to know the bank's name. If this error occurs with Citibank, please contact our Customer Support Team. They're currently investigating this issue, together with the bank, so we can fix this. Simply click the Help and select Contact us. Enter the exact error message in the question field and select Let's talk. Then, you can choose either Start a message or Get a callback.
There is one thing we need to check if you've encountered this error with a different bank. Make sure that they doesn't have any system issue. This could the reason why QuickBooks we'rent able to detect your bank. I suggest logging in to their website and see if there's a notifications or alert about it. Then, go back to QuickBooks and click the Update button on the Banking tab.
Let me share to you another way of bringing in the bank transactions. With the use of WebConnect feature, we can manually import the transactions. All we need is the CSV, QFX, QBO, or OFX files from your bank. Once you have it, you can follow the steps in this article: Upload transactions into QuickBooks Online.