Here is a test where you get the error:
import subprocess
ip = '192.168.15.5'
ipDest = '\\' +ip
subprocess.Popen(['psexec', '/s', ipDest, 'cmd'], shell = True)
Error presenting:
'psexec' is not recognized as an internal command or external, operable program or batch file.
Since the parameters are correct because they normally execute directly in the CMD.