Help creating a variable as a result

[buran]

bmi is float, and you compare it with a lists - under_weight, normal_weight, over_weight. They will never be equal.
In this case you don't need lists, they will not do what you want anyway.
one way to do what you try is like this:

if 14 <= bmi < 19:

by the way, bmi = weight / Height ** 2 is enough, no need to explicitly cast to float. Also it's good to be consistent with respect to variable names - use height, not Height. These are also PEP* recommendations.