I believe you are running your code within the interpreter, either using an IDE or the python prompt. This will not work as sys.argv relies on calling the program from the terminal command line. When I do so your code works correctly,
[myself@localhost ~]$ python f2c.py 12
12 degrees farenheit is equal to -11.1111111111 degrees Celsius.
To run it as ./f2c.py you need to do two things,
edit your code to resemble the following
#!/usr/bin/env python
import sys
f = int(sys.argv[1])
print (f, "degrees farenheit is equal to", )
print (5.0/9*(f - 32), "degrees Celsius.")
This is a command for linux shells such as bash to call the 'env' program and tell it that we need to use python to run it
Secondly, we need to make the file executable, this is done as
chmod +x f2c.py
Now you can call as follows
[myself@localhost ~]$ ./f2c.py 12
12 degrees farenheit is equal to -11.1111111111 degrees Celsius.
BTW, the correct spelling is Fahrenheit.