Problem when loading map

[Oscar House]

Here is where your problem is:

Spoiler below!

void OnLeverStateChange(string &in EntityName, int alState)
{
if (alState == 0)
SetSwingDoorLocked("mansion_1", false, true);
PlaySoundAtEntity("", "unlock_door", "mansion_1", 0, false);
}
void OnStart()
{
PlayMusic("01_amb_darkness.ogg, false, 1.0f, 0.5f, 10, false)
}

void SetEntityConnectionStateChangeCallback(leverbooks_01, OnLeverStateChange);
//===========================================
// This runs when the player enters the map
void OnEnter()
{
}

//===========================================
// This runs when the player leaves the map
void OnLeave()
{
StopMusic(0.5f, 1);
}

You shouldn't put it as a function, instead place it inside Onstart(), like this:

Spoiler below!

void OnLeverStateChange(string &in EntityName, int alState)
{
if (alState == 0)
SetSwingDoorLocked("mansion_1", false, true);
PlaySoundAtEntity("", "unlock_door", "mansion_1", 0, false);
}
void OnStart()
{
PlayMusic("01_amb_darkness.ogg", false, 1.0f, 0.5f, 10, false);
SetEntityConnectionStateChangeCallback("leverbooks_01", "OnLeverStateChange");
}

//===========================================
// This runs when the player enters the map
void OnEnter()
{
}

//===========================================
// This runs when the player leaves the map
void OnLeave()
{
StopMusic(0.5f, 1);
}

You also missed some quotation marks and a semicolon from there, I added those as well. See how it works, though I suspect there might still be something I missed.