not being able to upgrade my operating system hence cannot install apps

The tags appended to your post suggest that you have an iPad Mini with iOS12 installed. The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.6.

Apple ended major update support for these models of iPad in September 2019 - although they continue to receive occasional “point” security updates, the most recent being released August 2022. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU capabilities and available RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

App Developers often remove older versions of their Apps from the App Store. You should note that it is the decision of the App Developer - and not necessarily Apple - as to whether or not an older version of a specific App may remain available.

Should you wish to make enquiries of the App Developer, this support page may provide appropriate guidance:

How to contact an app developer - Apple Support

Ask the developer. They decide which OS versions to support, as well as whether older releases remain downloadable.

If you haven't downloaded it from the same Apple ID before, try that on a newer model.

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