Not sure why the variable isn't becoming defined in my if / elif statement

Question

I'm currently taking a 100 day python course and I am at the BMI calculator part 2. Here is the code I created:

height = float(input("enter your height in m: "))
weight = float(input("enter your weight in kg: "))
BMI = round(weight / height ** 2, 1)
if BMI < 18.5:
    category = underweight
    if 18.5 >= BMI <= 25:
     category = normal_weight
    elif 25 >= BMI <= 30:
     category = slightly_overweight
    elif 30 >= BMI <= 35:
     category = obese
    elif BMI > 35:
     category = clinically_obese
print("Your BMI is " + str(BMI) + ", you are " +category+ ".")

This is the error I am getting :

  File "main.py", line 18, in <module>
    print("Your BMI is " + str(BMI) + ", you are " +category+ ".")
NameError: name 'category' is not defined
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