The designer cannot be modified while debugging
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
This message appears when an attempt is made to modify items on the O/R Designer when the application is running in debug mode. When the application is running in debug mode, the O/R Designer is read-only.
To correct this error, select Stop Debugging on the Debug menu. The application stops debugging, and you can modify items in the O/R Designer.