I appreciate you reaching out to the Intuit Community. I can help you get your bank transactions in QuickBooks Online (QBO).
May I ask what bank institution you're trying to download in QuickBooks? This way, I can check if there are any open investigation about it.
Meanwhile, the error occurs if there are connection issues between your bank and with QuickBooks. As a workaround, you can download the transactions from your bank's website and manually upload it in QuickBooks.
Here are the steps:
Download transactions from your bank's website.
Login to your QBO, click Banking from the left menu.
Click the Update drop-down.
Choose File upload.
Click Browse to find and select the file to upload.