Creo NC, "can't access list of available post processors, exiting"

I don't know what other settings you have in your config.pro file, but these are my settings.

! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
! Category : Manufacturing
!

drilling_ui_end_opt_default reference
drilling_ui_start_opt_default reference
enable_classic_nc_steps yes
enable_classic_cutline yes
enable_classic_strghtcut_srfml yes
gpostpp_dir c:\ptc\Gpost
mfg_assign_param_formula no
mfg_ijk_num_digits 5
mfg_xyz_num_digits 7
mfg_setup_dir c:\ptc\MfgParams
mfg_solid_tool_display yes
moduleworks_setup_file c:\ptc\DefaultFiles\moduleworks-settings.xml
!nccheck_type moduleworks
nccheck_type nccheck
ncl_file_extension ncl
ncpost_type gpost
pro_mf_cl_dir c:\ptc\NCFiles
pro_mf_param_dir c:\ptc\MfgParams
pro_mf_tape_dir c:\ptc\NCFiles
pro_mf_tprm_dir c:\ptc\MfgParams\Tools
store_tool_path_file no

If I had to guess I'd say you are either not specifying a directory for the gpostpp_dir setting, or the location specified does not exist, or the directory specified is unreadable or empty. Maybe your setting points to a previous Creo installation directory that no longer exists?

There seem to be a bunch of "default" posts (Located in C:\Program Files\PTC\Creo 9.0.9.0\Common Files\x86e_win64\gpost on my machine) provided in the Creo installation. I don't know if they are any good - I've had my own locally stored list of posts for decades.