Some custom ROMs or modified versions of Lollipop misreport the SDK level. Also, devices without an ARMv7 or higher processor (very old Intel Atom chips) may not work.
The easiest and safest way to download WhatsApp is through the official store: Open the on your phone. Search for "WhatsApp" . whatsapp free download for android 5.1.1
If you cannot access the Play Store, you can download the installation file (APK) directly from the official source. How To Install WhatsApp App On Android - Step By Step Some custom ROMs or modified versions of Lollipop