iPad 1 can't install a new app
The original iPad (iPad 1) can support iOS 3.2 up to iOS 5.1.1 - source here: the iOS Support Matrix.

According to the link you provided for the app, the app you want to install requires iOS 8 or above.

This means that you can not install the app on your iPad, since it requires iOS 8, but your iPad can only run up to iOS 5.1.1.