I'd like to help you connect your bank account to QuickBooks Online (QBO). Can you share with me what bank you're trying to connect with? This way, I'd be able to check here on my end if there are any related issues. Additional information is much appreciated.
In the meantime, let's manually upload your bank transactions to your QBO account. Here's how:
Sign in to your bank or credit card's website.
Download your transactions to your computer.
Then, go to the Banking or Transactions menu.
Select Upload transactions.
Click Browse and select the file downloaded from your bank and select Next.