Developer tells me I can’t use dropdown in this context

I have a question regarding the attached picture, where I can choose one or more lines on screen 1 and make my choice on screen 2 allowing the "next" button to become available (in screen 1 it is gray because, in order to proceed, you must choose one or more options); As a result, in screen 3 I have an array of options and I start my session in screen 4, when I tap on option "30" and "start" becomes available (again, "start" is gray in screen 3 because you must select an option).

question 1: would you use an alert or any other warning panel that tells you that you MUST select something if you try to tap the gray status of "next" and "start"?

question 2: Developer says that I cannot have a situation like that, is either full screen or an action sheet. Is that correct? Really an option panel that act as a dropdown (screen 3 and 4) cannot be implemented? I have seen this kind of panel before without going full and used it as a solution here, but I can't remember where right now (I don't think an action sheet is the right choice in this context btw).

Any thoughts? Any feedback is greatly appreciated. Sorry if my explanation is a bit sloppy, I am still learning.