1.Restart the phone and see if you fixed the issue.
2.Go to [Settings] > [App Management] and find Camera. Click Camera, and then clear its data and cache.
3.Go to [Settings] > [Security] > [Permissions]. Click [Apps], click the third-party camera app you want to use, and then allow it to use camera.
4.Update your phone to the latest version.
Source: Realme