While development on Xcode using free development account, and if you are using real devices to install and test your apps and not the slow simulators, sooner or later, you are going to see the following error message:
The maximum number of apps for free development profiles has been reached.
The solution is quite simple and straight forward to keep using device for testing.
Go to iOS device and delete the one or all existing development apps that you have built.
Go to General > Device Management and remove the profile too.
Now try to install the app from Xcode.
hope that helps.