I can't purchase an app: it says unable to purchase this app is incompatible with this iPad. How do I fix this, I have iPad mini 2?

What is there to fix? The message says it all. Incompatible. Either look for a similar-function app that works with your tablet or replace your tablet to work with that app you want to purchase.

Many app has to update to go with newer models, opposite to that, not many customers will have older versions of th iPad so they stop supporting it, like the OS. You'll have to upgrade your iPad to access it.

You buy an iPad that the App is compatible with.

If you'd thought to tell us what the specific App is and which version of macOS your iPad mini 2 is running we MAY have been able to advise otherwise.