Line 14: You're using the assignment operator, not a comparison operator.
Your if statements are wrong.
Lines 14-15: This calculation is applied unconditionally, but only for the first 20kwh.
Lines 16-17: This calculation is for kwh>20 and <40 and excludes the first 20kwh.
Lines 18-19: This calculation is for kwh >40 and <50 and excludes the first 40kwh.