Picking List Registration Mobile App - The sync failed due to an error


Im developing a mobile app for D365 F&O -

a list of picking lists and details form for it with lines, details and overview form works fine, i add an action while being in freshly opened picking list details form WMSPickingRegistration, open the form -> click add action -> dont click add fields + button -> click on update action pane tab -> click update all -> update the picking list -> click 'back' in actions mobile app workspace button. So i get an action to update picking list, but then i try using it in mobile app details form for the picking list, i get this error in 'The sync failed due to an error'.

Why my app isnt working and how do i translate such errors into more understandable form ??