ERROR_REFERECE.md: adding how to handle ModulNotFoundError

Author: jimin-kiim

ERROR_REFERENCE.md

@@ -0,0 +1,13 @@
+## When an error occurred while installing packages with pip
+#### - `Requirement already satisfied: {package name}` but `ModuleNotFoundError` occurs when executing the code. 
+- It's saying that the installation of the package on your computer is succcessfully done. But, the specific location where it's installed might not be as we intended. 
+- Please check the path of the python kernel where the program is running and the one where the package is installed. They must be the same.
+
+    ```
+    where the program is running
+    $ import sys
+    $ print(sys.executable)
+
+    where the package is installed
+    $ pip show {package name}
+    ```