An iPhone 7 Plus is limited to iOS 15, and newer versions are not supported. To get to iOS 17 and iOS 18, you will need an iPhone XR., iPhone XS, iPhone SE 2nd, or newer.
But…. If you want help with a Brother printer, which specific printer model?
Depending on the particular Brother model, you may not need the Brother Mobile Connect app, which is the app you are probably trying to download here. That app won’t work as far back as an iPhone 7 Plus.
If the printer or multi-function supports AirPrint, that path can usually be used and with no added apps.
Alternatively, the Brother iPrint and Scan app supports iOS 15 and later, and can potentially be used here.