Ideally, you can connect as many accounts as you'd like in QuickBooks Online banking. Since you're having trouble doing so, let's clear your app's cache as the start of our troubleshooting.
To do so, simply go to the Help menu in your QuickBooks Online desktop app and select Reset App Data. But if still, it doesn't allow you to connect your bank or credit card, you can consider using one of our supported browsers instead.
Learn more about the browser app you can use for the best experience: Find out what computer and browser you need for the best QuickBooks experience.
Additionally, you can also browse for the content of this helpful article so you can easily categorize your bank transactions: Learn how to review bank and credit card transactions after you download them.
If you have any other questions, please let me know in the comments. I'll be here to help. Stay safe!