Normally, running this command would give you the help documentation, but as you can see it errors out. The same happens when I try to use afni_proc.py with specific inputs a la my old scripts.
Replying to my own comment here, but for those using MacOS and MacPorts, you can switch the default python to 2.7 (assuming you have it installed) using:
The
National Institute of Mental Health (NIMH) is part of the National Institutes of
Health (NIH), a component of the U.S. Department of Health and Human
Services.