I figured it out. The URL scheme is taken form the project name. My project name had an underscore in it, which Apple doesn’t like. I changed it to a hyphen, but then unreal called it an unknown symbol on packaging. So I took it out completely and made it on alphanumeric name and Apple accepted that.
Now that we know this is an issue, maybe Unreal can fail to build when it encounters illegal characters in an ios build, or autocorrect them?