In your StructureGroup constructor call you are passing it a TCM URI of a Folder (item type 2), but the method requires the TCM ID of the parent Structure Group where you want to create the new one, I.e. Item type 4). See here for a reference of item types: http://tridion.stackexchange.com/a/3015/159https://tridion.stackexchange.com/a/3015/159
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