I have had an app and now I can’t download app again. It keeps saying “unable to download”.

Okay,

If the app already exists on your iPhone,

Go back to your older iPad, open the iOS App Store, go to the Purchases section of the store app, search for the app/s you wish to try and download/re-download to that older iPad and see if an older version of the app is allowed to be downloaded/installed to that older iPad.

These newer iOS/iPadOS devices must use your same Apple ID user account and password.

Failing that OR IF NO joy, then there is nothing more to be done.

You will NOT be able to retrieve the older version of this app as the developer/s of the app may have remove their older versions of their app/s from Apple's iOS App Store servers.

Best of Luck to You!

A little hard to advise and help you when you provide NO information about your iPad.

Can you confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in small print, right under the larger bold print “iPad”.

What iOS/iPadOS version is this iPad, currently, running?

All of that additional info can be found in the Settings app, General settings panel, About.

Also, can you confirm what year and model iPhone you are using and what iOS version it is currently running?

Please attempt to answer all questions asked.

Thank You.