Can’t log WhatsApp after upgrade iPadOS 15.7

Rolling-back to an earlier version of iOS/iPadOS is not possible.

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

If Apps no longer work properly following update to a more recent version of the OS, you’ll need to seek assistance from the related App Developer - as only they can resolve issues with their App.

How to contact an app developer - Apple Support

Consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data. 

In the meantime, consider using a different Web Browser App to access WhatsApp - such as Firefox:

https://apps.apple.com/gb/app/firefox-private-safe-browser/id989804926

Safari is an integral element of iOS/iPadOS - and cannot be updated independently of the OS. 

If your iPad has been updated to the most recent version supported by your iPad (the model and installed version of iOS have not been declared), then Safari is as up-to-date as is possible.

However, some websites incorrectly identify your iOS/iPadOS Safari User Agent when used in one or other of its browsing modes - of which their are two - these being Desktop and Mobile. It’s easy to switch between modes if a website isn’t functioning correctly. From the desired web-page, touch the aA icon at the left of the Address Bar; this will reveal a menu. From the menu select Website Settings, then select (or deselect) Request Desktop Website, then tap Done. Now refresh the webpage.

The chosen setting will be saved for the selected website - and can be simply reversed if needed.

If you continue to experience difficulties, you may have more success by using a different web browser App - such as Firefox:

https://apps.apple.com/gb/app/firefox-private-safe-browser/id989804926