Can I make the same app for iPad which is already lives on AppStore for iPhone? I had created an app which is live on AppStore for iPhone only. Now I want to make that app for iPad. So can I make the same app for iPad or do I need to create the separate app on AppStore Connect for that?

As I remember 4 years ago, I got this problem in one of my application due to which I was not able to make my app for iPad. And I had to create the separate iPad app at that time.
The answer depends on what you mean by "making the same app for iPad".

If you mean that you're going to take the app (its source code) and make it compatible with both the iPhone and the iPad at the same time - and then publish that on the App Store for both platforms - then yes, you can do that. When you change your Info.plist to indicate that you support both iPhone and iPad as targets and submit your app update, the App Store will automatically list the app for both types of devices.

If you mean that you're going to create a separate app (new project) that will support the iPad only, and you want to publish that under the same name as the iPhone-only app - then no, you cannot do that. You would need to publish that under a different name.