Thank you for posting here in the Community. I'm here to help you add a bank account in QuickBooks Online.
You have two options in adding an account in the Online version. You can add it through the chart of accounts or in the Banking section (download transactions automatically).
For the chart of accounts, here's how:
Click the Gear icon in the upper-right corner.
Select Chart of accounts.
Press New in the upper-right corner.
Enter the necessary information.
Hit Done.
If you want your transactions to download automatically, I recommend adding the bank account through Banking. Please follow these steps: