You are unable to open K2 for Visual Studio projects.
1. An unsupported .NET Framework version is installed and it does not allow you to open K2 Projects.
2. K2 for Visual studio Client Components are missing in Blackpearl Setup Manager > Modify K2 blackpearl.
To resolve this:
1. Make sure that you only install supported versions, as per our compatibility support matrix: https://help.k2.com/k2compatibilitysupportmatrix
It only supports .NET Framework 4.0/4.5 (RTM) or .NET Framework 3.5 (SP1).
2. You will need to run the setup manager from the installation media to install K2 for Visual Studio Client Components. Please follow the link below for more information: http://help.k2.com/onlinehelp/k2blackpearl/icg/4.7/default.htm#install/manual/Farm_BP_SelComp.htm