Why i cannot install any app on my ipad

There are many possibilities - but without more details of the issue and any errors that you see, it is difficult to provide a definitive answer. The tags appended to your post suggest that you have a first generation iPad mini - with iOS9 installed.

A first generation iPad mini 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 iPad mini in September 2016. The iPad mini1 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; iPad mini1 is a 32-bit architecture device, whilst all recent versions of iOS/iPadOS are designed for 64-bit hardware platforms.

You may wish to consider that compatible Apps for older models of iPad are increasingly difficult to find; 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. As such, with an older model iPad, 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.

If your issue goes beyond that of availability of compatible Apps for an old-model iPad, this support page may be helpful in addressing a technical problem:

If you can’t download or update apps on your iPhone or other Apple device - Apple Support

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