The route from Cancun to Istanbul is a new route for Turkish Airlines (I was recently on a Turkish flight where the crew was talking about it excitedly, as a nice new place for them to go) and service is set to start in August.
New international airline routes typically require the approval of the relevant governments, and can be the subject of trade negotiations. I suspect this message is meant as a notice that the Mexican government has not yet approved the new route. They wouldn’t have put the flight on the schedule if they didn’t believe the approval was forthcoming, but I’m not certain what will happen if you go ahead and book now. There’s at least some small possibility the flight could be cancelled if they don’t receive the necessary approvals in time, though the time between August and October gives you a decent buffer.
Credit:stackoverflow.com‘