That’s probably because you’ve converted your input to string with the String from Object node. At that point it’s a string and trying to divide it by some value won’t work. The output of your code block is probably a list of nulls.
You should first try to find out what kind of unit revit expects as input. You can do that like this:
Look up the pressure_Loss parameter in that list and its corresponding storagetype. If it’s a double or integer than you have to feed the input of you Setparameter node a numeric value.