Andrew, due to warranty concerns, especially on a brand-new pistol, I would task Ruger with getting this corrected at Telephone: 336-949-5200 / (Monday through Friday 8:00am - 5:00pm ET). Cheers and stay safe!

Andrew, unfortunately this sounds like a common issue with the Ruger security 9. I'd recommend trying to loosen the slide release by moving it up and down until it loosens up. If it's really stiff it could take a couple hundred times of doing so to get it to work properly. Also according to Ruger the Security-9 is equipped with a slide lock rather than a slide release. As Ruger instructs in the owner’s manual, the proper way to release the slide is to pull it back and let it go to release the lock and let the slide snap forward to load a round into the chamber.
However, some owners report that the slide will still not release when pulling back and letting the slide go unless they manually hold the slide lock down at the same time. If that's the case you'll likely have to contact Ruger for warranty work. You can contact Ruger customer service at (336-949-5200) Monday through Friday 8:00am - 5:00pm. Sorry you're having trouble but hope this helps some.

Andrew, not sure if this will help, but look here >>> rugerforum.net/threads/security-9-slide-release-fix.396306/ ....also see 'Difficulties with the Slide Lock/Release' section here >>> https://thegunzone.com/worst-ruger-security-9-problems/