This is an "oldy" but "goody". When you add a new parameter in the Family Types dialog and then immediately attempt to use that parameter in a formula Revit generates this message.
To avoid this message just click APPLY or OK (which closes the Family Types dialog box) to commit the addition of the parameters to the family. Then return to the Family Types dialog and add your formula. You just need to let Revit get used to the new parameters first, then it will happily let you make the formulas you need.