Unsubscribe and resubscribe to the mission, should fix it. I tested it from scenarios menu and it works fine. If all else fails you can also fire it up from local server.

Am I doing something wrong? I click play and nothing happens , no errors , nothing. The play button is just non reactive.

I know it took me some time, but I finally updated the mission. Single player experience should be easier and more enemies will spawn in multiplayer, depending on player count.

As for AI going through walls, mission takes place in vanilla location, not something made by hand, the issue should be non existent. AI will do its thing sometimes tho, its arma 3 after all.

If you ever decide to replay the mission I hope you will have a great time and would appreciate your feedback on that.

in your last 2 defense missions, often the enemy just walks through the walls directly towards the VIP or the player. I played without extra mods.
In general, these 2 missions are too difficult. It's impossible to survive at the veteran level... You have to make the right balance! Making impossible missions is too bad a trend...
I was able to pass it only from the 5th time and only 1 character from the squad survived. And the corpses are disappearing too fast. , weapons cannot be picked up.

and another one xD