Why my iPhone 6s Plus can’t install after downloading

Hi.

The most obvious would be compatibility issue. And the other one would be security restrictions by default on the phone settings. If you downloaded the application from the AppStore, then you should not have any problems since this is screened by Apple and will be installed automatically. If it's the latter, then you need to go somewhere in the settings, bottom pane, look for the app and allow it to run. Warning, this might damage your device. Only allow if you know what you are doing and the application or developer is trusted. Read more here https://support.apple.com/en-ca/HT207165.

see if anything in this user tip helps

Updating iOS: common issues and solutions - Apple Community