download whatsapp on my apple watch and no notifications from imessage

It sounds like you don't have notification for WhatsApp enabled on your iPhone/Apple Watch.

Open Settings on your iPhone

Scroll down and tap on Notifications

Find WhatsApp

Turn on Allow Notifications

Open the Watch app on your iPhone.

Tap Notifications

Scroll down to the Mirror iPhone Alerts From section

Find WhatsApp in the list and toggle it ON