"Part xxxx is an enforced piece part.
Components cannot be added to it."
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'Enforced Piece Part' makes part as piece part so that other parts/components can not be added to it. Generally it is used for the standard parts like fasteners.
File > Utilities > Enforced Piece Part.
Clear the check 'Enforce as Piece Part' in the dialog 'Piece Part Enforcement' and OK. Now you can add component/part to it.