compatible issue

You cannot restore/install an earlier version of iOS/iPadOS than is currently installed on your iPad.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new or updated iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple for a short period - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

If the App with which you are having difficulty has not itself been updated to be compatible with the currently available version of iOS/iPadOS - perhaps because the App has been abandoned by the App Developer - you’ll not be able to use the App with your iPad.

Your only option is to contact the App Developer - and enquire as to their plans to update the App. You will usually find a link to the Developer’s support pages from the associated page in the App Store. Otherwise:

How to contact an app developer - Apple Support

riming64 Said:

"compatible issue: old ipad cannot buy & install intune company portal app"

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Troubleshooting iPad Installs:

Being so old of a device, it's likely no more than an issue with the hardware incapable of running this software. So, if this is for a company, then ask the Network Administrator what the minimum system requirements are.