Recently there has been a major issue of WhatsApp suddenly not installing on android tablets. It keeps saying "Your device isn't compatible with this version. WhatsApp is designed to be a Mobile messaging app, even though they allow users to send and receive messages on Computers, they are still keeping the installation process limited to mobile phones. That is the reason you cannot install WhatsApp on your tablet from Play Store.
I am going to show you two methods to have whatsapp up and running on your tablets.
1st Method: Go to settings -> Application Management -> select Play Store and clear cache and data. After restarting, try downloading whatsapp now.
2nd Method: If the first method did not work, go to WhatsApp.com and download the apk file from WhatsApp website and install it. Now, if you open whatsapp site on your tablet, you will be redirected to the play store when you tap on the download link.
You can either open it in Chrome browser of your tablet and choose desktop site from the footer, and then click on the download link Or you can download from this link:
Click on the downloaded .apk file to install. Make sure you have checked the 'install from unknown sources' option in Settings.
After installation, go through the usual process of setting up, and you are good to go!