WhatsApp was heavily developed for Windows Phone 7.5, 8, and 10 . These OS versions are entirely different from the kernel used in Windows Mobile 6.1/6.5.
If you are determined to use these legacy devices for communication, they are best viewed as digital time capsules. While they can no longer connect to the modern WhatsApp network, they still excel at: whatsapp messenger.cab for htc windows mobile 6.1
or WhatsApp Web, though these still require a linked, compatible smartphone to function. WhatsApp Blog alternative messaging apps WhatsApp was heavily developed for Windows Phone 7