I'd be happy to help you fix this. There are times that this is caused by the outdated data in your browser. Let's perform the browser troubleshooting steps to figure it out.
First, you'll have to log in using an incognito browser and check if you can successfully add your bank from there. If you can add your bank, you can go back to the regular browser and clear the cache. If not, use another supported browser.
You may check out this article if you're using QuickBooks App through your mobile phone to add your bank. This contains troubleshooting steps to help you get back on track: Troubleshooting Quick-Start Guide for the QuickBooks Online app. This article is also applicable to QuickBooks Self-Employed App aside from QuickBooks Online.
In case you want to know some "How do I" steps in QuickBooks Self-Employed, you can always visit our Help Articles page for reference.
Please don't hesitate to let me know if you have other questions or concerns about QuickBooks. Take care and have a wonderful day.