$ cwltool --debug missing.cwl 
INFO /home/michael/wdl2cwl/env/bin/cwltool 3.1.20220204090313
INFO Resolved 'missing.cwl' to 'file:///home/michael/wdl2cwl/missing.cwl'
DEBUG Parsed job order from command line: {
    "id": "missing.cwl"
}
DEBUG [job missing.cwl] initializing from file:///home/michael/wdl2cwl/missing.cwl
DEBUG [job missing.cwl] {}
DEBUG [job missing.cwl] path mappings is {}
DEBUG [job missing.cwl] command line bindings is []
DEBUG [job missing.cwl] initial work dir {}
INFO [job missing.cwl] /tmp/6lbyzlz3$ 
ERROR Exception while running job
Traceback (most recent call last):
  File "/home/michael/wdl2cwl/env/lib/python3.9/site-packages/cwltool/job.py", line 344, in _execute
    rcode = _job_popen(
  File "/home/michael/wdl2cwl/env/lib/python3.9/site-packages/cwltool/job.py", line 982, in _job_popen
    sproc = subprocess.Popen(
  File "/usr/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.9/subprocess.py", line 1691, in _execute_child
    executable = args[0]
IndexError: list index out of range
WARNING [job missing.cwl] completed permanentFail
DEBUG [job missing.cwl] outputs {}
DEBUG [job missing.cwl] Removing input staging directory /tmp/z5ivv_hu
DEBUG [job missing.cwl] Removing temporary directory /tmp/cuirlvtk
DEBUG Removing intermediate output directory /tmp/6lbyzlz3
{}
WARNING Final process status is permanentFail