Verify that an iCloud backup exists in WhatsApp > Settings > Chats > Chat Backup.
If you can see when the last backup was performed, delete and reinstall WhatsApp from the App Store.
After verifying your phone number, follow the prompts to restore your chat history.
You must be signed in with the Apple ID you use to access iCloud and iCloud Drive must be turned on.
There must be enough free space on both your iCloud and iPhone. You need to have at least 2.05 times the space available in your iCloud account and on your phone than the actual size of your backup.
The phone number used for backing up and restoring must be the same. You can't restore the chat history from another WhatsApp account.
Even if you share an iCloud account with a family member, you'll still be able to keep your WhatsApp backup separate from theirs due to the requirement of needing the same phone number for both your iCloud backup and WhatsApp account.