i have the PRO plan and, in order to show a customer a preview of his app, i published as Web App.
In my mind it was just temporary.
After a couple of days i realized i just have 2 Web App available in my PRO plan and, at the moment, i would like to unpublish one and get the slot back.
At the moment, the option to have responsive web apps is available only to Business or Team Level Users. With one of these subscriptions, you will be able to publish your app.