CharacterEnteredTrigger and GlobalEventCleared are both Events that fire when something happens. The game can't check whether two events happen simultaneously (or in sequence) directly, so you need to create an intermediate boolean/flag after the first event in the sequence completes.

Code
IF
GlobalEventCleared("LakeVisited")
THEN
DB_LakeVisited(1);


IF
CharacterEnteredTrigger(CHARACTER_Player_01,TRIGGER_LakeTrigger)
AND
DB_LakeVisited(1)
THEN
//Stuff