If you get a message like this, especially with the text "Sometimes the files are locked by VS - please try again.", the most likely solution is to close all tabs in VS that show generated code. For example, close all tabs and restart VS to be sure no files are locked.

If you happen to have a Solution Items node in your project - you should remove it. Not sure who put it there but it has been reported that Codegen stumbles if this is present.

See also: Code generation not working, Codegen Issues .net standard