oh yes ! you didn't install mitmproxy x)
version 1.8.3#3 right ?
1) Open Powershell
2) paste this: (Invoke-WebRequest
... | python -
3) paste this: [Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python27\;C:\Python27\Scripts\", "User")
4. paste this: pip install mitmproxy
it will install mitmproxy