You might experience some problems with your Google Play application. To fix it, try going to the menu of your device > Settings > Applications > Manage Applications > All tab, tap on Google Play and select Clear Data and Clear Cache. After you done this, try downloading and installing WhatsApp again. Also, it's advised to use an official source to install applications onto your device, so it's better that you always use Google Store. If you still experience troubles with WhatsApp, try contacting the developer on the following mail address : email@example.com. Maybe, he can provide additional support in this matter.