When naming parameters in your Revit project or family files, we should try never to use symbols in the name. These include the addition ‘+’, subtraction ‘-‘, multiply ‘*’, divide ‘/’, and even the bracket ‘()’ symbols. Revit will allow you to add these symbols to the name but problems can follow later if you start to write formulas.
When you start to create formulas, this is where Revit will start throwing up errors and you will end up not being able to save your dialog box. The names conflict with Revit’s mathematical processes.
So to be safe, try never to add symbols to your Parameter names.
P.S. There are other mathematical statements that can cause conflicts too, like ‘if’and 'not'……