Just a note, the entire error should be posted from the log into the report:
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
at System.ThrowHelper.ThrowArgumentOutOfRangeException (System.ExceptionArgument argument, System.ExceptionResource resource) [0x00029] in <567df3e0919241ba98db88bec4c6696f>:0
at System.ThrowHelper.ThrowArgumentOutOfRangeException () [0x00000] in <567df3e0919241ba98db88bec4c6696f>:0
at QuestActionSpawnEnemy+<SpawnEnemies>d__5.MoveNext () [0x000ed] in <01ad48046e714d799bcf4dc03d3db7ff>:0
at UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) [0x00027] in <5e1dfb686eb345078a588df96401afcc>:0
(Filename: <567df3e0919241ba98db88bec4c6696f> Line: 0)