In order for WhatsApp to use the photos from your Camera Roll or Photo albums, you'll need to give the app permission to use the Photos from your iPhone. If you declined permission to use photos, you'll see this alert:

You can grant permission by changing your iPhone's privacy settings:

1. Go to iPhone Settings > Privacy.
2. Tap Photos > WhatsApp > Read and Write.

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

If WhatsApp is greyed out or doesn't appear in privacy settings

This is a rare issue, but it happens to a small number of users. Ensure that you don't have any restrictions set in iPhone Settings > Screen Time. Otherwise, you'll most likely need to backup and restore your phone.