-getsoftlist requires the name of a software list as parameter. So the following will work:
mame64 -getsoftlist apple2_flop_orig
That's for current MAME, older versions might differ.
Also the -listsoftware parameter is probably what you are looking for here - it will go through all softlists and depending on the machine chosen will output what has been deemed able to run.
> mame64 -listsoftware apple2e
This show all software XML which is valid for that machine. If you want individual setnames and descriptions (similar to -listfull), you'll need to parse that info yourself from the text output capture.