When trying to use the builtinparameter enumeration, you have to use
.GetParameters() uses the string that is on the right side (description). Right now, yours is trying to find a parameter that doesn’t exist so it returns an empty list.
It is generally recommended to use .GetParameters() as it returns a list of all parameters with the same name, where get_Parameter only returns the first one it finds. But this also means that .GetParameters() returns a list so you have to either take the first one and hope there are no duplicates, or do some logic/testing to take only the one you want.
Here is an example: