The WhatApp developers have not released an App compatible with iPad - the App being currently limited to iPhone.
Whilst the App is not available for iPad - you should be able to access the service using a web browser:
https://web.whatsapp.com/
To use the service, you will still require a mobile phone (not necessarily an iPhone) with which to associate the device that is accessing the service from a web browser.

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added an eSIM or physical DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number cannot be used for making calls or sending messages.