I know a couple of reasons why you're unable to connect your bank account to QuickBooks Online (QBO), info-dinnerwiths.
I'll provide them below and guide you through connecting them seamlessly.
Before anything else, I'd like to ask for the name of your financial institution. This way, we can verify if your bank is supported with QBO.
Otherwise, having issues with linking your bank account may be because of the following:
Multiple names for the bank on the list.
Bank not supporting all account types (e.g., personal vs. business)
Bank not being a participating financial institution.
You can request support from your bank if your bank isn't supported. Here's how:
Go to Transactions, then select Banking.
Hit Connect account.
Search for the name of your financial institution. If QuickBooks can't find your bank, you will get the message, "Hmm, we can't find [name of the bank you entered] in our list of supported banks," and presented with helpful tips to try and locate your bank account.
If you still can't find your bank, click Request.
Enter your bank's web address (URL) in the field provided, then select Request.
In the meantime, you can still download your transactions from your bank's website and manually upload them into QBO. Please see this article for the detailed process: Manually upload transactions into QuickBooks Online.