Back Camera not working after screen replacement?

mary Cynthia, it's possible both issues are caused by the same problem of the camera not working. It's quite common for the flash to be disabled when an issue is detected with a rear camera, so fixing the camera will probably fix the flash.

Generally a screen replacement shouldn't have any effect on the rear cameras since they shouldn't get disconnected in a normal replacement procedure. However, take a look at Step 21 of the iPhone XS Max Screen Replacement Guide.

You'll see that you have to disconnect the front sensor assembly in that step. Note that the connector is the one right next to the connector for the lower rear camera as shown in this picture.

My best guess is that the camera connector got bumped loose when unplugging the front sensor array and is no longer plugged in correctly. I'd suggest opening the phone back up and unplug and reconnect both of the rear camera connectors to make sure they're seated securely and there's no damage to the pins on either the cable or the motherboard, so unplug them and look at them carefully to make sure they're okay. The iPhone XS Max Rear-Facing Cameras Replacement Guide will show you which connectors to look at in Step 28. Clean them with 90% or higher concentration isopropyl (rubbing) alcohol, then plug them back in. reassemble the phone far enough to be able to power it in then retest the rear camera and see if it's working.

If it is, great, button the phone back up and call it a day! If not, come on back here and let us know what you found and we'll see if we can figure out what your next step should be.