In order for WhatsApp to use the photos from your Camera Roll or Photo albums, you will need to give the app permission to use the Photos from your iPhone. If you declined permission to use photos, you will see this alert:
You can grant permission by changing your iPhone's privacy settings:
- Go to iPhone Settings > Privacy.
- Tap Photos.
- Make sure WhatsApp is set to ON.
Open WhatsApp, and you should now be able to use your existing iPhone Photos with WhatsApp.
If WhatsApp is greyed out or does not appear in privacy settings
This is a rare issue, but it happens to a small number of users. Ensure that you do not have any restrictions set in iPhone Settings > General > Restrictions. Otherwise, you will most likely need to restore your phone. This Apple Support Page explains the process: iOS: How to back up and restore your content.
WhatsApp Support Team