You need to have more karma points on your profile. The downvote privilege has been at around 500 karma points (for a long time). That point is only the threshold, and the privilege is granted after going above that threshold. But it supposedly increases over time. The Unofficial Hacker News FAQ [1] has a lot more information.

[1]: https://www.jacquesmattheij.com/the-unofficial-hn-faq

After users reach 501 Karma, they gain the ability to downvote another comment

You can read more about undocumented features of Hacker News here [1].

[1] https://github.com/minimaxir/hacker-news-undocumented

You need to reach a certain karma threshold, I don't remember the exact value.