Text Animator error: Can not “set expression” with this property...

Hi All.

I have a problem with an animator. I am trying to change the expression of one of the properties via scripting, and I get the error: "Can not “set expression” with this property, because the property or a parent property is hidden."

However, the animator was made by script and I could apply an expression at the time that I made it. If I make a new one, I also can change it.

Is there anything I could do to make the animator not be "hidden" anymore? I am not even sure what this "hidden" is.