For the user to access the camera app in the Multi-app kiosk, you can add the app to the multi-app kiosk policy. Navigate to Android Kiosk Lockdown-> Multi app in the kiosk policy and add the camera app there. Please note to associate targets and save the policy.
If you do not want the app as itself to be added to the kiosk but wants the user to be able to access the camera app functionality from other apps in kiosk, you can add the camera as a background app. To add background apps, please navigate to Android Kiosk Lockdown-> Background Apps, Click on the ‘+ icon’ and add the desired app. Click here to know more about the background apps.