In order to use your photos and videos on WhatsApp, you’ll need to give the app permission to access the Photos on your iPhone. If you declined permission to use photos, you'll see this alert:

Grant permission

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

Ensure that you don't have any restrictions set in iPhone Settings > Screen Time. Otherwise, you might need to backup and restore your phone.