Keys on Doors...

[TheGreatCthulhu]

Just for the sake of clarity: he didn't forget to declare
mansion_2 - he forgot to put it in quotes, and make it a string literal (that is: "mansion_2" ).

The compiler, reaching the token mansion_2 saw that it wasn't in quotes, so it assumed it's a variable, but it couldn't figure out what kind of variable it was, since such a variable was never declared.
A declaration is when you introduce a variable to your code, like this:

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string targetDoor = "mansion_2";

This declares a string variable named targetDoor, which can then be used later on in various places in the code (assuming it's a global var), like this:

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PlaySoundAtEntity("", "unlock_door", targetDoor, 0, false);