Sumar dos campo de medida / Add two Measure fields (it is not a column)

Tengo dos campos de medida que cree, ahora necesito sacar la diferencia etre los dos, es decir una operacion matematica y me da un mensaje de error, ya que dice que no pueder realizar la operacion con un dato de texto.

I have two measurement fields that I believe, now I need to make the difference between the two, that is to say, a mathematical operation and it gives me an error message, since it says that I can not perform the operation with a text data