After a lot of search and reading posts, here's what fixed it for me.
It the Targets>[your app]>General tab, I had not yet set my app icon name in the App Icons and Launch Screen section yet. It had placeholder text "App Icon" that I incorrectly read as "AppIcon", which is what mine is called.
Once I added that the problem was fixed.
Topic:
App Store Distribution & Marketing
SubTopic:
TestFlight