Newt had already retired by that time.

Though Newt is still alive during the Harry Potter series, he never meets Harry (and therefore readers never encounter him in person) because his life doesn’t really intersect with Harry’s. At the time Harry was introduced to the wizarding world, Newt would’ve presumably already retired. It’s mentioned in the “about the author” section of Fantastic Beasts and Where to Find Them, which is one of the assigned books in Harry’s first year, that Newt’s now retired.

“Newt Scamander was awarded the Order of Merlin, Second Class, in 1979 in recognition of his services to the study of magical beasts, Magizoology. Now retired, he lives in Dorset with his wife Porpentina and their pet Kneazles: Hoppy, Milly, and Mauler.”
_{^{- Fantastic Beasts and Where to Find Them}}

As a retired man living in Dorset, Newt wouldn’t have had much chance or reason to interact with Harry.

However, Newt is mentioned as an author.

Though Newt never appears in person in the Harry Potter series, he is mentioned in it, as being the author of his book Fantastic Beasts and Where to Find Them, which is one of the required books for Hogwarts during Harry’s first year.

“Set Books
All students should have a copy of each of the following:
The Standard Book of Spells (Grade 1) by Miranda Goshawk
A History of Magic by Bathilda Bagshot
Magical Theory by Adalbert Waffling
A Beginner’s Guide to Transfiguration by Emeric Switch
One Thousand Magical Herbs and Fungi by Phyllida Spore
Magical Drafts and Potions by Arsenius Jigger
Fantastic Beasts and Where to Find Them by Newt Scamander
The Dark Forces: A Guide to Self-Protection by Quentin Trimble”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 5 (Diagon Alley)}}

He was a presence in the wizarding world of Harry Potter’s time, though he wasn’t physically present in the series. It’s also clear from this that Newt’s book is still important and useful to the wizarding world several years after he originally published it, so his work is still very much present. As Albus Dumbledore mentioned in his foreword for it, Fantastic Beasts and Where to Find Them has been an approved textbook at Hogwarts since its publication.

“I WAS DEEPLY HONOURED when Newt Scamander asked me to write the foreword for this very special edition of Fantastic Beasts and Where to Find Them. Newt’s masterpiece has been an approved textbook at Hogwarts School of Witchcraft and Wizardry ever since its publication and must take a substantial amount of credit for our students’ consistently high results in Care of Magical Creatures examinations – yet it is not a book to be confined to the classroom. No wizarding household is complete without a copy of Fantastic Beasts, well thumbed by the generations who have riffled its pages in search of the best way to rid the lawn of Horklumps, interpret the mournful cries of the Augurey or cure their pet Puffskein of drinking out of the toilet.”
_{^{- Fantastic Beasts and Where to Find Them}}

Newt himself was retired by then, but he was present in the Harry Potter era wizarding world because of his book, which was still widely used in the wizarding world, as well as being one of the textbooks required for Hogwarts during Harry’s time there.