While building a new Web Deployment project, I received the error "Unable to build project output group 'Content Files from (Active)." I checked all of my settings and everything seemed to be correct.

I found a support article on Microsoft's Help and Support site describing why this occurs. Apparently, if you have missing files in your project this will happen. In this case, it turns out that a folder of images had been moved in the source tree but not in the project file. This was causing them to all show the the yellow exclamation symbol. Even though the project has "warnings as errors" turned on, the compiler will not complain about missing content files since it does not need to do anything with them.

If you get the error, quickly scan the project you're deploying for any missing files. It will be a file with the Build Action set to Content.