It might be an issue with the system set-up. The most common reason is that the coordinate breaks for your tilted surface were added manually instead of using the tilt/decenter tool (and therefore the surface is oriented incorrectly or the thickness didn’t change sign, or other rules are broken). The easiest fix is often to remove your coordinate breaks and then use the tilt/decenter tool to recreate your scanning tilt.

Hi @Zeinab.Sanjabieznaveh - to extend on Alissa’s point, here is a tutorial on creating a scanning mirror system: How to model a scanning mirror – Knowledgebase (zemax.com). Let us know if you are still having any trouble after applying these tips .