I don't see why GetFieldSelections() won't work. I guess create a KPI object with GetFieldSelections(Ano) to see if it show anything (make sure you set custom format to see the text values, else KPI object might just show -.
Also try GetSelectedCount(Ano) may be that one works, if it does, you can use that.
Also Try to check what does this give you:
If(Len(Trim(GetFieldSelections(Año))) = 0 and Len(Trim(GetFieldSelections(Mes))) = 0, 1, 2)
All these are ways I can think of troubleshooting the issue.