Hey, you wrote some code! Credit for that. :-)
But what did you do?
This function has NO outputs! None at all. It dumps somethign to the command window, but that is all. It does NOT return a result.
But then, how do you use it?
you use it, expecting an output.
What was the error message?
"too many output arguments"
So you wrote a function that returns no arguments, but then you used it expecting an output argument. Therein lies the problem. The fact is however, you are not using any output from isPrime, and it already dumps a comment to the command window.
Just change the code you use to this:
starting_point=input("Enter your number: ");