First sense: natural as belonging to nature's first institution
First, what is present from nature's first institution is called natural. In this sense he says that immortality was natural to the first man and death against nature, since man was created immortal at the beginning, that is, free from the necessity of dying.
Latin
XIV. Primo enim naturale dicitur quod inest a prima institutione naturae. Et hac ratione dicit immortalitatem fuisse primo homini naturalem, mortem vero contra naturam, quoniam homo initio institutus est immortalis, id est, a moriendi necessitate liber.