Dear Fredy,
I think the issue is related to the frequency window setting (sample rate) in the project layout parameters window. You need to make sure that this window include both signals to allow the simulation.
You may email me your project as a zip file to check it out. My email is ahmad.atieh@optiwave.com.
I need to know the version of OptiSystem that you use and your institution name. Also, if you have valid maintenance and support license.
regards,
Ahmad