Can I install whatsapp on iPad Air 2 cellular wifi model

Hi ramanand. The app requires a phone with voice and SMS capabilities. Your cellular iPad only supports data. It is not a phone, just a mobile data device.

When the developer will have a whatsapp version for iPad with cellular model ?

You are going to have to ask the developer of WhatsApp. No one here would have any idea when that might happen.

Personally my guess is that it will never happen.

NO, as the developer has not created a version for the iPad.