I presume Apple has changed the layout of iTunes Connect and the location where you add in-app purchases, so this is the screen where I add in-app purchase, however it tells me to Select it from the app’s In-App Purchases section and click Submit.
Which again I presume is the following area, which I can't see any where to submit in-app purchase:
According to this question and it's answer, there should be an in-app purchase part but it is missing.
P.S. This is going to be the first submission for the app.