Today I crossed this error when building a solution from Visual Studio 2015. The signtool.exe is a tool which is used to sign applications (or dlls) so it can be traced back to a specific certificate. When you don’t use it, you don’t need to know but the error can be annoying.
There is a simple solution though; edit your Visual Studio settings and make sure the ClickOnce components are selected, see below:
With these installed, signtool.exe should be a part of your deployment and you’re now good to go. Enjoy!