local variable 'bm' referenced before assignment

This error occurs when you try to use a local variable before it has been assigned a value. In Python, variables are created when you assign a value to them. If you try to use a variable before it has been assigned, Python does not know what value to use and returns an error.

To fix this error, make sure that you assign a value to the variable before using it. For example:

def calculate_bmi(height, weight):
    bmi = weight / (height ** 2)
    if bmi < 18.5:
        return "Underweight"
    elif bmi >= 18.5 and bmi < 25:
        return "Normal weight"
    elif bmi >= 25 and bmi < 30:
        return "Overweight"
    else:
        return "Obese"

height = 1.7
weight = 65

bm = calculate_bmi(height, weight)

print(bm)

In this example, we assign the result of the `calculate_bmi()` function to the variable `bm` before using it in the `print()` function. This ensures that the variable has a value before it is used.