GeoGen Script reference
GGE2602

Specified key was not found in an array.

This error is by the [] operator in read mode and functions like Array.RemoveKey.

var array = {"a": 1, "b": 2 };
var c = array["c"]; // GGE2602
array.RemoveKey("a"); // OK
array.RemoveKey("a"); // GGE2602, this key is already deleted