A little utility to run inside Maya's script editor (Python tab!) to troubleshoot ngSkinTools installation issues.

ngSkinTools_diagnostics.py

print "-------------------------------------------"
import sys
import os
from maya import cmds

print "Environment info:"
print "Maya version:", cmds.about(installedVersion=True)
print "Maya API version:", cmds.about(apiVersion=True)
print cmds.about(operatingSystem=True), cmds.about(operatingSystemVersion=True)

print

ngstPath = [i for i in sys.path if "ngskintools" in i.lower()]

print "Checking python path:"
if ngstPath:
    print "ngSkinTools found in python path:"
    print "\n".join(map(lambda x: " * "+x, ngstPath))
    from ngSkinTools import version
    print "UI version:", version.pluginVersion()
else:
    print "ngSkinTools not in python path"

print

 

print "Checking plugin path..."
ngstPluginPath = [i for i in os.getenv("MAYA_PLUG_IN_PATH").split(os.path.pathsep) if "ngskintools" in i.lower()]

if ngstPluginPath:
    print "ngSkinTools found in plugin path:"
    print "\n".join(map(lambda x: " * "+x, ngstPluginPath))
    
    print "loading plugin..."  
    result = cmds.loadPlugin('ngSkinTools')
    print "plugin loaded:", cmds.pluginInfo('ngSkinTools', q=True, path=True)
    print "plugin version:", cmds.pluginInfo('ngSkinTools', q=True, version=True)
else:
    print "ngSkinTools not in plugin path"


 

print

print "opening ngSkinTools UI..."  
from ngSkinTools.ui.mainwindow import MainWindow
MainWindow.open()
print "done."

print "-------------------------------------------"

sample expected output.txt

-------------------------------------------
Environment info:
Maya version: Autodesk Maya 2018.4
Maya API version: 20180400
win64 Microsoft Windows 8 Home Premium Edition, 64-bit  (Build 9200)


Checking python path:
ngSkinTools found in python path:
 * C:\ProgramData\Autodesk\ApplicationPlugins\ngskintools\Contents\scripts
UI version: 1.7.6

Checking plugin path...
ngSkinTools found in plugin path:
 * C:/ProgramData/Autodesk/ApplicationPlugins/ngskintools/Contents/plug-ins
 * C:/ProgramData/Autodesk/ApplicationPlugins/ngskintools/Contents/plug-ins/2018
loading plugin...
# Warning: Plug-in, "ngSkinTools.mll", is already loaded.  Skipped. # 
plugin loaded: C:/ProgramData/Autodesk/ApplicationPlugins/ngskintools/Contents/plug-ins/2018/ngSkinTools.mll
plugin version: 1.7.6

opening ngSkinTools UI...
done.
-------------------------------------------