Other: "You know the app you're going to develop for us?"

Me: "Yes."

Other: "Can you give me access to it so I can take some screenshots for writing the guidance?"

Me: "Err.. Well not really as it doesn't actually exist yet..."

Why do I have to deal with such people.

Maybe because these ppl want to parallelize the processes.

If it happens often: try some prototyping tools (like axure - there a more just Google what fits best )
You can first "drag drop" the gui, implement very fast minimal logic (login, click here brings me there) and show how it will work.
- customer /manager can see and proof of it will work
- designer can start the real work
-backend can do the Logic easier
- handbook /guide can be already made - of the gui elements stay more or less the same (e g. Same place/shape)

- less misunderstandings

@Daniil great idea in theory, but given they haven't even finished providing requirements anything I prototype would be based on guess work and incomplete knowledge.

@LynxMagnus
Ohw... Well... No words then