In order to answer your question it is first of all important for me to explain to you what an antagonist is. The main character in a story is called the protagonist. For example, Harry Potter is the protagonist of the Harry Potter novels, as he is the main character in these books.The antagonist, on the other hand, is the person who acts against the interests of the protagonist. Simply put, the protagonist in a story is usually the hero, whereas the antagonist in a story is most often the villain.

Sticking with the example of the Harry Potter novels, you could therefore say that Lord Voldemort is the antagonist in these novels. Lord Voldemort, often also referred to as "the Dark Lord" or "You-Know-Who", sees Harry Potter as a threat to his own power in the world of wizards. This is supported and backed up by Sybill Trelawney's prophecy in Harry Potter and The Order of The Phoenix:

The one with the power to vanquish the Dark Lord approaches .... For neither can live while the other survives.

The fact that Harry's mere existence poses a threat to his power is the reason why Voldemort tried to kill Harry when Harry was only a baby, which, as a result, led to the tragic death of Harry's parents.

I hope this explanation makes it clear to you why Lord Voldemort can be regarded as Harry Potter's antagonist: he is Harry's main enemy and opponent. Given that Harry is the protagonist of the novel, this makes Voldemort the antagonist.