Huh? There is no mistake here. The routing allows you to fly from CUN-GDL *IF THERE IS A FLIGHT* . Since there is no flight on that allowed routing , then you cannot book it. if an airline starts/restarts service on that routing then you will be able to book it

There is no reason whatsoover why the airline should allow it; there is no mistake, just that there is no valid flight that you can use

Dave