A quick clarification: have you actually loaded the marines into the Ospreys via the cargo menu, or are they just equipped with a "Marines, 24x" loadout? You mentioned were new to the game, and this would be the kind of thing that would (understandably) confuse new players. If this is the issue, see this thread here for additional details: https://www.matrixgames.com/forums/tm.asp?m=4889552
(Then again, you did say you played the cargo tutorial, so presumably you already know this. But it doesn't hurt to ask.)
Regardless, thewood1 makes a good point.
(I'm also assuming, based off of the above linked thread, that this scenario does indeed use the Chains of War cargo features. It's worth pointing out, even if it doesn't necessarily apply here, that many older scenarios, that were made pre-CoW, don't. They usually involved teleporting in the ground units once the transport entered a defined zone. Just something to watch out for.)