If you have used the keyword mandatory for the attribute "Posicion" in the AttrRep of the class "Puerta", the field is displayed in red only when the table is empty (standard value). As soon as you have a row in the table, the attribute name is black, as the current value of the attribute "Posicion" and the default value (empty) are different. Unfortunately, the "mandatory" keyword has no effect in the AttrRep definition of the RecordClass used for the Table attribute.
However, since the RecordClassMultiplicity is set to 1, meaning that you can only have one row, why not define two separate attributes, x_Posicion and y_Posicion, and use the keyword "mandatory" for them? These two attributes may be placed inside a GROUP, called "Posicion" and the AttrRep code would look like this:
1NOTEBOOK
2...
3CHAPTER "Description"
4GROUP "Posicion"
5ATTR "x_Posicion" mandatory
6ATTR "y_Posicion" mandatory
7ENDGROUP
8...