I don't think you did anything wrong. I think you'll find that everyone has the same experience when reinstalling wmc after updating to Windows 10 1803. wmc can be started from the start menu or desktop shortcut with open or run as administrator, but like the behaviour first seen with Windows 10 1709 there are problems running it full screen. These can be resolved to some extent by running in compatibility mode for Windows 8, and disabling fullscreen optimization. However, even with these modifications wmc now is sluggish and in full screen mode will freeze now and then while navigating around the menus. With Windows 10 1709 the freezing problem in full screen was resolved by copying the ehome directory from Windows to Program Files and running wmc from there in compatibility mode and disabling fullscreen optimization. However that fix is not working with Windows 10 1803. There is no difference in behaviour between running wmc from the Windows or from the Program Files directories. I have gone back to 1709 and blocked feature updates for now in the hopes that someone comes up with a new fix for running wmc in full screen stably in 1803, or possibly some future windows security or feature update makes that possible.