What do I need to do when my phone tells me “Unable to verify this iPhone has a genuine Apple camera”?

It would be a good idea to have it looked at by Apple. If it is a brand new phone (not just new to you) then it should not be displaying that message.

Genius Bar appointment - Unfortunately Apple has made this tricky to get to directly. Go to https://www.apple.com/retail/geniusbar/ > Get hardware help. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair" --> Sign in with Apple ID

I experienced this same issue and I took my phone (12 Pro) to Apple. They advised my phone had "unauthorized modifications" and they could make no repairs. Which makes no sense given that the phone is still under warranty, has had no issues to this point and as never been to anyone -- Apple nor an "unauthorized" third-party for repair or replacement of any part. I purchased this phone brand-new directly from T-Mobile and received it wrapped and boxed in Apple packaging. While this may not impact the functionality of the phone currently, this is an issue that may impact my ability to get future repairs if this phone does experience a true functionality issue. This business practice in place may be working as Apple intends, but certainly not correctly if a phone was never serviced/modified by anyone. This shortchanges customers and places them a precarious position in the event an issues emerges that would be covered under warranty that cannot be covered due to the warranty being erroneous (and potentially deceptively) voided by the determination of incognizant Apple technicians.

See this document --> About Genuine iPhone cameras

This seems to be an issue with a run of iPhones sold by T-Mobile. I would contact their customer service and see if they can help you out.

I bought my phone directly from Apple and am having the same issue.

So then you made an appointment with Apple and dealt directly with them? So what happened?