my iPad shows the message " Unable to Purchase" This apps is not compatible with this iPad

You need to understand that your very old first generation iPad2 can only be updated to iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular models).

Apple ended update support for iPad2 in September 2016. Your iPad cannot be updated to iOS 10 or later major versions of iOS as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. This limitation cannot be bypassed; iPad2 is a 32-bit architecture device, whilst all recent versions of iOS/iPadOS are designed for 64-bit hardware platforms. iPad2 is obsolete.

Compatible Apps for your iPad are increasingly difficult to find - as third-party developers are increasingly dropping support for older versions of iOS preceding iOS11/12/13. As older versions of Apps are removed from the Apple App Store, you’ll not be able to (re)download or install them - which may severely limit the usefulness and utility of your iPad. As such, if you want or need to access most current Apps, requiring more recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model.

Of current models, even the entry-level iPad8 has specifications and capabilities that will massively outperform you current iPad - and is compatible with the vast majority of current Apps. Other models are more powerful still.

I hope this information provides helpful and succinct clarity as to your available options - and the limitations of your current iPad.

Your over 9-year old, 2011 iPad 2 is too old.

8 and 9-year old iPads had their support ended by Apple over 4-years, ago.

Third party app developers ended support for these 8 and 9-year old iPads over 2 years, ago.

There are virtually NO MORE apps for these 8 and 9- year old iPad models, any longer.

Your iPad is too old and obsolete, now.

Sorry.

If you need/want and iPad that can run a current iOS/iPadOS version with current, updated apps, then you need to seriously consider a new iPad model OR a much, MUCH “newer” , used iPad model capable of running the latest iPadOS versions.

There are plenty of newer, older iPad models that can run the current versions of iPadOS 14.

Sorry & Best of Luck to You!