Just backing up jconsol's solid advice. :O))
I'm guessing you weren't using the official website. As jconsol said, you couldn't buy tickets as they are only on sale through September 15th. Their tours, should you want one of those, are reasonably priced at 25 euro (including reservation fee) per adult.
No idea when Sept 17 tickets will be available as the ticketing vendor hasn't been releasing them on a consistant schedule lately. Just check daily to see when the date you want shows as active for purchase?
https://www.tosc.it/en/artist/galleria-borghese/galleria-borghese-2253937/#calendar-start=2024-08
As a side note, the link below is for the museum's official info website; it has a portal that connects to the ticketing site linked above if you click on the pink "Buy" on top right of the page but you can skip the two-click process to get there by just going directly to ticketing via the links provided. This site is only accessible during the hours, Rome time, the museum is open but ticketing is "up" 24/7.
https://galleriaborghese.beniculturali.it/en/