created Android app cannot be installed on real device, Testing on that device works

[Colin Holgate]

You will get exactly the symptoms you describe if you install an app with the same ID but that has had its certificate changed. It could be that the certificate persists, and the system knows that the ID is already associated with a certificate that it has. But also, if you're not using a certificate, I wonder what happens if there is already an app installed that also doesn't have a certificate?

If you do a Google search of 'android deleting old certificates' you'll see lots of questions about removing old trusted certificates. Might be worth checking out.