I'm wondering if you installed a second version of Python when you installed pythonwin. Its been some time, but I seem to recall a prompt from pythonwin at install time asking about using existing Python or a new install. From your traceback image above, I see the path 'C:\Python27\Lib\site-packages\pythonwin.....' which I don't think should be.
In your pythonwin Interactive Window, try the following and share the printout:
>>> import sys
>>> sys.path
['C:\\', 'C:\\Windows\\system32\\python27.zip', 'C:\\Python27\\ArcGIS10.6\\DLLs', 'C:\\Python27\\ArcGIS10.6\\lib', 'C:\\Python27\\ArcGIS10.6\\lib\\plat-win', 'C:\\Python27\\ArcGIS10.6\\lib\\lib-tk', 'C:\\Python27\\ArcGIS10.6\\Lib\\site-packages\\Pythonwin', 'C:\\Python27\\ArcGIS10.6', 'C:\\Python27\\ArcGIS10.6\\lib\\site-packages', 'C:\\Program Files (x86)\\ArcGIS\\Desktop10.6\\bin', 'C:\\Program Files (x86)\\ArcGIS\\Desktop10.6\\ArcPy', 'C:\\Program Files (x86)\\ArcGIS\\Desktop10.6\\ArcToolBox\\Scripts', 'C:\\Python27\\ArcGIS10.6\\lib\\site-packages\\win32', 'C:\\Python27\\ArcGIS10.6\\lib\\site-packages\\win32\\lib']
>>> sys.executable
'C:\\Python27\\ArcGIS10.6\\Lib\\site-packages\\Pythonwin\\Pythonwin.exe'
>>> import os
>>> os.listdir(r'C:\Python27')
['ArcGIS10.6']
>>> os.listdir(r'C:\Python27\ArcGIS10.6')
['DLLs', 'Doc', 'include', 'Lib', 'libs', 'LICENSE.txt', 'NEWS.txt', 'python.exe', 'pythonw.exe', 'pywin32-wininst.log', 'README.txt', 'Removepywin32.exe', 'Scripts', 'tcl', 'Tools', 'w9xpopen.exe']
import sys
sys.path
sys.executable
import os
os.listdir(r'C:\Python27')
os.listdir(r'C:\Python27\ArcGIS10.6')