Ok so in your init.sqf file you have the following correct?
 execVM "briefing.sqf";
Then the briefing.sqf you might want to change to something like this:
Just a tip, but i always find it best to make sure any object "names" whether it be markers or units are done without a space, in reference to your "VC mortar" marker name.
task1 = player createSimpleTask ["OBJ_1"];
task1 setSimpleTaskDescription ["Eliminate Vietcong Mortar", "Eliminate Vietcong Mortar", "Eliminate Vietcong Mortar"];
task1 setSimpleTaskDestination (getMarkerPos "VCmortar");
task1 setTaskState "Created";