aah, I see what the problem is, "--gui-mode simple/expert" has been removed.
This is according to the changelog on the web site.
https://github.com/prusa3d/Slic3r/releases
The following features were removed:
The "simple" mode has been removed as it was too simplistic, therefore nobody used it.
The "--gui" option is still there, but if you call it with "--gui-mode <simple/expert>" it just gives the calling instructions.
I would suggest you remove the "--gui-mode" part or tell us where you add the "--gui-mode" part and we can remove it ourselves?