From the link:

You might have added an assembly or a manifest file as a content file. You should use the Add Reference command instead of Add File so that the dependent assembly is properly referenced by the project system. More sophisticated projects commonly mark the .exe.manifest in the bin folder as a project file, but you should avoid doing so. The hidden app.manifest file becomes the .exe.manifest file and can be edited manually for advanced scenarios

>Yes, the link showed only what the problem was and not how to fix it.
>I found the solution myself which I want Microsoft to write as ....
>
>Steps to get rid of apps.manifest
>1) Open the project and click 'View', 'Solution Explorer'
>2) By default all the files are NOT listed, so make sure to click 'Show All Files'
> (Here is IT professional's common problem in assuming everyone knows this.)
>3) Select 'My Project'
>4) Select apps.manifest (without 'Show All Files' this link is NOT shown.)
>5) Delete the link.
>
>I want this type of step by step instruction!