Tafsir of Al-Jinn 72:21

Say: "I possess no power to cause you harm or bring you to right guidance." That is, I possess no power to cause you harm or bring you benefit. This is an expression using the name of the cause for the effect; the meaning is: "I am unable to harm you or benefit you; the only One who harms and benefits is Allah, the Almighty and Majestic."

Or it may mean: "I possess no power to lead you into error or guide you to what is right," where 'harm' is intended to mean 'error,' using the name of the cause for the effect. This is evidenced by the recitation of Abu [al-Ala]—'ghayy' (error) instead of 'darr' (harm)—and the meaning is: "I am unable to compel you toward error or right guidance; the only One capable of that is Allah, the Exalted."

It is also possible that the verse contains ihtibak (ellipsis/omission), the original being: "I possess no power to cause you harm or bring you benefit, nor to lead you into error or guide you to what is right," where one element from each pair is omitted because of the other. Al-A‘raj recited 'rushudan' with two dammahs.