I'm in the same situation - nearly - I'm trying to get an app into TestFlight, not do an Ad-Doc distro. I get the same 'private key not installed' error. I'm using XCode 10.3 - so maybe not a XCode bug, @Vinterlord. I can see in KeyChain Access that my certificates definitely do have the private key installed (I can see it when I expand the certificate). I've been through many forum answers that recommend clicking 'Manage certificates' and creating a new iOS Distro certificate (I get the error "You already have a current iOS Distribution certificate or a pending certificate request"), or downloading the certificates from the developer portal and reinstalling them on my machine. There are other suggestions but they seem to be out of date. If I try manually signing the app, I eventually receive an 'App Store Connect Operation Error' because my app 'is not signed using an Apple submission certificate'