The compiler finds a mission label that is not expected there based on the mission declaration order. It may also happen if you put an external script between two missions. The external scripts must follow the missions.
Possible solutions: check out if the order of missions in the file matches the header. One of the missions is out of order:
DEFINE MISSIONS 2DEFINE MISSION 0 AT @MISSION_0DEFINE MISSION 1 AT @MISSION_1:MAIN:MISSION_1 // error, MISSION_0 expected:MISSION_0