The Cubiscan 210SS uses a few different sensor types to perform a measurement. If the red sensor beam does not turn on/shine on the belt as a package runs through the system, the most common cause is a misaligned photo eye trigger at the entry end of the system. Check for proper alignment with the reflector on the opposite side of the belt. This Photo Eye sensor detects a box entering the system, and must be functioning properly. Proper alignment will cause a Green and an Amber/Orange light to light up on the top or back of the sensor.
If the Photo Eye Trigger is aligned, and the beam still does not turn on when a package passes through the system, locate the power cord for the SICK equipment, and unplug the dimensioner system. Wait at least 20 seconds, and then plug the system back in. This will reset the system, and should resolve the issue.
If the recommendations to this point do not resolve the problem, check the rotary encoder sensor that is mounted on the belt below the scale/conveyor. If this speed sensor is not contacting the belt and spinning properly, this will also prevent the system from activating. 
If none of the above solutions resolve the problem, deeper troubleshooting is needed. Contact Cubiscan Support for troubleshooting assistance and ensure that a TeamViewer or other remote desktop application is available for them to work through to troubleshoot the equipment remotely.