I replaced my screen on my iPhone XR now my front camera is showing black

Who replaced the screen? If you or a 3rd party iFixit shop put an aftermarket screen on your phone, they messed it up. Take it back to them. Apple won't touch a phone which has been tampered with aftermarket parts.

Now, if Apple replaced the screen you have a guarantee from Apple, so take it to them. But again, if Apple didn't replace the screen you're unfortunately on your own.

K. iPhone is not a user serviceable device. You clearly messed the phone up. Since you can't take it to Apple for service, find a iFixit type stand, like you likely have at any mall in your area and see if they can help you.

Note: with a 3rd party screen on your phone, Face ID will likely never work on the phone.