After a few weeks I returned to a Windows Universal app in Visual Studio and hit F5 to run and debug it. It said:

Registration of the app failed.
Another user has already installed an unpackaged version of this app.
The current user cannot replace this with a packaged version.
The conflicting package is {NAME} and it was published by CN={GUID}. (0x80073cf9)

Normally you just click the Windows start button, type the name of the app to find its name and icon, then right-click Uninstall. That had no effect. I tried running as my normal user and Administrator but it had no effect.

Run Powershell ISE with administrative rights. Run Get-AppxPackage -all and you should see the offending package near the end of the long list (you can use filter arguments or pipe the output into a filter, but I forget the syntax). Look for the PackageFullName and pass it into this command:

Remove-AppxPackage -Package PackageFullName

After doing that I could run and debug in Visual Studio again.