Make sure you have enough storage on your phone.
Download Whasatpp https://www.androidapksfree.com/apk/whatsapp-apk-latest-version-download/
go to Settings > Security/Applictaions > and check the "Unknown Sources" check box (to allow non-Play Store applications to install). The launch the "File Manager" application or any other File Explorer/ Manager application your device is running. From there select the SD card storage location > open the "Download" folder from there > scroll to and tap the downloaded Whatsapp apk file > scroll through the terms and conditions and then tap "Install".