[Fixed]-Error while executing python manage.py runserver

You appear to be mixing up paths.

Generally, if you type python, the system looks up your pythonpath, and resolves it to the python executable. On the windows systems I’ve used, that tends to be at c:\python35\python.exe

However, in this instance, you’ve given it a full path, but not actually pointed it to the executable.

I believe (assuming your path is correct) that this version should work:
C:\Python35\Scripts\mysite\python.exe manage.py migrate.

That said, on my windows machines, I can just do
manage.py migrate, because the python.exe is set by pythonpath when Python3.5 was installed.

Tl;dr:

Run manage.py migrate or use a full path to the executable.