If you are enrolled in the Windows Insider program and have been hit with this message when trying to update to 1903 and none of the various other Googled solutions work, try this one from Windows support forum user “LenniL”:

It isn’t enough to simply uninstall the games that may be using this anti-cheat software, you need to manually delete the folder.

For good measure, you might want to delete other anti-cheat software that may also still be left around.