I have mono installed from beta channel (http://www.mono-project.com/docs/getting-started/install/linux/) - version 4.0.4
I can't compile Visual Studio 2015 project.
In project options under Build->General the Target framework is .NETFramework 4.6 (Not installed).
You will have to downgrade to .NET 4.5 profile if you do want to target both .NET and Mono, or you help Mono out by filling the gaps between 4.5 and 4.6, or you simply wait till someone else helps out.
Updated: This answer applied to the Mono release at that time. Latest Mono release does support .NET Framework 4.6 and above, so you don't need to downgrade any more.