PyCharm "no module named sys"

I had this issue and I resolved it by updating the default project settings under File -> Default Settings -> Project Interpreter and then in the drop down list for Project Interpreter, selecting the same interpreter as the Run Configuration was using (in my case python 3.6)

Make sure you've chosen the right Interpreter path and check some other run settings. Go to Run > Edit Configurations... and check, if some configurations seem to be wrong.

EDIT: Under Linux, you have often Python 2 and 3 installed, be sure to pick the correct one.

I had this problem as well and was completely stymied until I tried the following:

File -> 
Default Settings -> 
Project Interpreter
  dropdown (click the arrow) ->
Show All ->
  select your interpreter ->
Show Paths
  (bottom icon on right) ->
Add Path ->
  Enter path to libraries
  (in my case this was c:\Python35\Lib).

I have had this issue with PyCharm before. I find that reloading the interpreter fixes the issue.

File -> Settings -> Project -> Project Interpreter

Remove the interpreter and add it again.