If you are running msbuild.exe via command line and get this error:
CSC : error CS1617: Invalid option '7.3' for /langversion; must be ISO-1, ISO-2, Default, Latest or a valid version in range 1 to 7.1.
e.g. from also from a build agent such as jenkins, the solution may be as easy as to upgrade Microsoft Build Tools 2015.
You can do that via choco install microsoft-build-tools or manually via the official Microsoft Build Tools 2015 or by updating your Visual Studio 2017 installation.
If you found this post useful, consider buying something fun/useful for yourself or a gift for a friend using my banggood referral link and I'll get a little commission (you still get the same -best- price). Thank you.