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:
Open WhatsApp, and you should now be able to use your existing iPhone Photos with WhatsApp.
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.