The mission is designed to be played with friends in multiplayer. It does not work to choose Singleplayer - Scenario to play it in Singleplayer.

But there is a way to play it alone.

1) Choose Multiplayer from the Main Menu
2) Choose Quick Play
3) next choose Host Server
4) you can keep the pre set options - make only sure to host in LAN
5) click on Host Server
6) Next Screen choose the Map Lybor and then the Mission
7) Click on Play
8) Now you will get the Role Selection Screen - all available slots should be occupied by AI
9) Choose the Team Leader role and click on OK
10) Now the game should load and you should see the Map
11) Click on Continue
12) Thats it you will spawn as the Team Leader and have control over the other 3 AI soldiers.

Have fun.