If you need to install Flash Player on Windows Server 2012 R2 (for example if you plan to use VMware vSphere Web Client), the solution is quite simple - just install (in fact - enable - as it's already installed, but disabled) the Desktop Experience feature under Server features.
This can be done:
a) by using the Server Manager (it can be found under "User Interfaces and Infrastructure" at the "Features" tab of the "Add Roles and Features Wizard")
b) by using the following PowerShell commands: import-module ServerManager
By the way, when using the vSphere Web Client, depending on the version, update of Internet Explorer and Flash Player might be needed.