The tags appended to your post would suggest that you have a long-ago obsolete iPad4.
Update support for iPad4 ended September 2017. The final iOS releases for this model were iOS 10.3.3 (WiFi Only models) and iOS 10.3.4 (WiFi & Cellular). iPad4 cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions. The 32-bit platform upon which the iPad4 is based is obsolete.
App Developers have dropped all support for 32-bit devices and many for iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store. Withdrawal of Apps supporting older models of iPad is making their utility and usefulness significantly more limited. Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.
If Apps that you need to use now have higher minimum iOS-version requirements than your iPad is capable, your only option is to replace your iPad with newer model. Of current models, even the entry-level iPad9 supports current iPadOS 15 - and has hardware specifications and capabilities that vastly outperform an old iPad4.
Apps aside, your iPad4 has an equally old version of the Safari browser. Modern websites will invariably use features and functionality that your browser App doesn’t support. As such, you can expect to encounter difficulties with many websites.