There‘s no iPhone 9. In 2017, Apple released the iPhone 8 and 8 Plus, as well as the iPhone X. Last year, the iPhone XR, XS and XS Max have been released. Which one is yours?
Whether or not you can use a jailbreak tweak, depends on the software installed on your iPhone. If it‘s up to date (iOS 12.4), it can‘t be jailbroken. There are only certain software versions that can be jailbroken.
If it‘s possible to jailbreak your device, or if it‘s already jailbroken: the Cydia Store has been shut down last year. It‘s possible that you can‘t get tweaks from there.
Applocker, the tweak you mentioned, works for iOS 6 to 10.2 only. So if you purchased your iPhone in 2017, you can‘t install it on your device. You might have to find a different one.
What exactly do you need the tweak for?