Expression Error: Undefined value used in expression (could be an out of range array subscript?)

Hi, 

I'm following this tutorial by ECAbrams https://www.youtube.com/watch?v=EG7m3j1X9hQ&t=207s.

I think I'm doing everything the same but I get this error in the screenshot. 
I wanna make a rectangle which refers to the size of the textbox. 

I didn't finish writing the expression because the error is in the first line.