Thanks for providing such detailed information, ankitdkabra.
Thanks for elaborating how you're unable to disconnect your bank account in QuickBooks Online. Also, the screenshot gives me a clearer view as to what's happening when doing the process. I can help get things sorted out for you.
I've checked if we have an ongoing issue on this, and so far, there's none. Just as the recommended solution by @MorganB above, we can do a basic troubleshooting steps to resolve this.
You can start by logging into a private browser. Please note that, browsers use cache of memory to work efficiently and load pages faster, however as data collects it can cause unexpected results.
The steps depend on which browser you're using:
Once done, restart your browser for the changes to take effect. These steps should help disconnect the bank account.
Let me know how the process goes once you get the chance to try it. I'll be around!