file doesn't exist

[buran]

Everywhere in the code you compare bytestrings with strings. So the file is never send

        if user_response[:2] == 'OK':

this code from SERVER will never be True

You need to fix this and elsewhere (if any) in the code where you make such tests (data received via socket)... the same way like the other case - either decode bytestring or compare with bytestring, nit string