• Have you cloned the right repo?

i download the latest source file tar ball/zip.

• Have you synced?

i presume you mean github… not using it.

• Have you made local modifications to the project files?

no.
i unpack the zip and go directly to the /PowerEditor/visual.net/ directory.

• Is there anything wrong with your VS install?

I doubt it, but couldn’t rule it out.
Last year-ish, i used the vs2005 project files provided and “upgraded” them to a VS2008 IDE, those typically worked out-of-the-box for 20 npp versions.

• notepadPlus.vcxproj - VS2013 project, used to build release binaries AFAIK, works well for me.
• notepadPlus.vs2015.vcxproj - VS2015 project, kept up to date AFAIK, presumably available as a courtesy for users who don’t have 2013.
• notepadPlus.vs2005.vcproj - probably outdated and should be deleted.

That error you’re getting doesn’t make any sense, because none of the project files has the text <IncludePath> in them. The error message points to the .props file - have you taken a look at it…?

Some things to check, off the top of my head:

• Have you cloned the right repo?
• Have you synced?
• Have you made local modifications to the project files?
• Is there anything wrong with your VS install?

If you’re saying VS2013 is required, why do these older things exist?

the 2015 solution errors:

Error: The element <IncludePath> beneath element <Project> is unrecognized. C:\Users\xxxx\AppData\Local\Microsoft\MSBuild\v4.0\Microsoft.Cpp.Win32.user.props

that’s not saying oh you got some include path wrong, thats saying, i don't even understand this file's format

What version of VC++?
What is the error message?

I think VS2013 is now required.