First of all, you need to delete your 1st line of code as in that line, you have simply mentioned the PI variable, but you have not defined that variable, so that is why you are getting this kind of error message.
Your function code is fine, but in your code, you need to calculate the area of the circle so for that the formula is Pi*R*R, so it will be ** 2, not * 2. You can also use the math package:
return math.pi * radius ** 2
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