Hi Jean! I had been busy and this thread took a backseat (like, end of the school bus backseat).
The path I used was leading to a folder I created in the root drive, on the desktop; for the test, it seemed to me like a quick and dirty but reliable way, following Mac "guidelines" as understood by casual users who put everything there, so I thought at least that would be a path that would be logical, easy to define and equally legitimate.
I have the full path displayed at the bottom of the window and I collected this access path with right click: Services > Copy path.
I tried all sorts of semantics, even using the ~ or not, trying absolute or relative writing, etc.
Didn't work.
The only one that worked thus far was the official Pictures folder, in Mac name > Users > [name of user] > Pictures.
It's actually the MyPictures (2) in the Screenshot action.
The two other options, File Path (1) and Use Default Folder (3), are set to None.
It's not a big big problem either. Don't anguish over it. :-)
At least one option does work nicely.
The question of enum menus in actions is a larger one, that of options that are obviously exclusive of each others (ergo only one option can be valid at a time). Even more since I recall 1.9 introduced a kind of dynamic UI for action, where an option changes a section of the action's block. That's a neat new [UI] function available but I'm not sure if many users rely on it.
It would certainly look sleeker but that's for another time.