In Sub-process Setup > Input Variables, all data to pass to sub-process is de

Are you trying to run the subprocess unattended? If so, change the assignment to "run as an automated activity". That should get rid of the form, and the dropdowns should appear.
If you are trying to run the subprocess attended, try removing the fields and mappings from the form. The dropdown should then appear for those removed form fields.

Thanks Bishnu!