Because you don't have any data files in there, indeed.
The pipeline editor and Brainstorm batch system can only have in input and output files containing signals (recordings, sources, time-frequency, scouts signals...). To be called from a script, the "Compute head model" process needs that you import of simulate some signals in there.
I guess what you are trying to do here is some simulation work on forward models only.
This very specific case can't be handled by the standard Brainstorm scripting system. If you don't have any recordings in the folders in which you want to compute forward models, you need either to do it from the interface, or to call the lower-level functions manually. Two options, depending on the level of details you want to provide:
-
panel_headmodel('ComputeHeadModel', iStudies, sMethod): Read the help at line 284 inpanel_headmodel.m -
bst_heamodeler.m: Put a breakpoint at line 673 panel_headmodel.m, observe what is in the input structure and reproduce it in your script.
Not that these are advanced undocumented features of Brainstorm: we can't guarantee backwards compatibility for these calls in the future, and we can't provide step-by-step help for using them. You need to be very confident with your Matlab programming skills to try either of these solutions.