We haven't received any reports from other users like you who are unable to add a new bank account for QuickBooks Online banking. Thus, let's perform some browser troubleshooting steps so you can successfully add you bank account.
To do that, let's pull up and access your account in a new incognito or private window of your browser to check if its a browser-related issue or not. From there, you can go to the Banking page and add your bank account.
Once you have successfully connected your account, you can clear your browser's cache to optimize your browser's performance. It'll also improve your browsing experience while working with your business in QuickBooks Online.
However, if you're still unable to connect your bank account, let's switch to a new browser application. And then, pull up and access your account and try to connect your bank account from there.