I think this is because some scrolls are scribed as Divine Magic, and others Arcane Magic, or Natural Magic dependent upon the class of the scribe (in this case an unknown NPC who's scroll somehow found it's way into the posession of the creature you've slain and are looting).
Rangers use Natural Magic, so if the scroll doesn't list Natural Magic specificly in the description when you examine it, or if the casting level or attribute requirements are not met by your character, then your character won't be able to cast the spell on the scroll.