[warning]Visual Studio version '14.0' not found. Falling back to version '15.0'
Since I do not install Visual Studio 2015 Express for Desktop and Web on my build agent, I am not sure whether Visual Studio 2015 Express for Desktop and Web is compatible with build agent.
But I found a thread about it, you can check the comment on the accepted answer:
That's it! For some reason VS2015 Express does not install the ShellFolder or InstallDir keys! I was worried that the agent script recognised it but refused to register it because we are not allowed to use it for automated builds. I feel more comfortable with it now.
It turns out that is not the full story. When I add the "Visual Studio
Build" task to the process, it now executes, but if I look more
closely at the log, I can see the following message:
"##[warning]Visual Studio was not found. Try installing a supported
version of Visual Studio. See the task definition for a list of
supported versions." And then it falls back to building with MSBuild.
It works because I am building a .sln; let's just hope I don't have to
build a .proj.
So, it seems that Visual Studio 2015 Express is not compatible with build agent. you can try to install the Visual Studio community 2015 instead of it, which I can build it without any issue.
Again, you can check if there is a parameter about Visual Studio on your Capabilities of build agent when you use Visual Studio 2015 Express:
if not, build agent will throw that error.
Hope this helps.
/p:VisualStudioVersion=14.0. If still have this issue, please create a new blank project and submit it to the build agent and build it, check the result. - Leo Liu-MSFT