How can I install Teams on iOS 9?
No. An application is (broadly) a list of instructions to the OS. Each new OS contains new instructions that applications can use.
MS Teams requires instructions that are only found in iOS11 and later. Your OS doesn't recognise these.
Some apps do keep older versions on the App Store for older OS/hardware to use: but I don't know the history of when Teams was ported to iOS.
An iPad2 is 9 years old. While you can always keep using the same software for as long as the hardware works (I'm surprised the battery's still going!), if you need newer software, then you have to have hardware that's closer in age.