How can I solve it?
The config supplied with GSX for that model it's an alias of the one for the 145LR, which *does* have its own staircase. If the XR model doesn't have one, you can customize it using the airplane configuration editor, and set the door's height correctly.
Note that, of the staircases supplied with GSX, the one that can reach the lowest height it's a ladder that reaches 1.85 meters so, the door must be higher than that, otherwise there are no staircases in GSX that can be used with that plane. I don't know how high the passenger doors is on that plane but, if it's lower, but not so much lower than 1.85, setting it to 1.85 or higher will allow the ladder to be selected, but might not be visibly noticeable, if the difference is a few centimeters.
