Locate the path of csc.exe and add it your PATH environment variable.
In my case, the path for 64-bit C# compiler is C:\Windows\Microsoft.NET\Framework64\v4.0.30319.
Similarly, you can look for 32-bit C# compiler in C:\Windows\Microsoft.NET\Framework under different .NET framework version directories
There will be csc.exe for all versions like v2.0.XXXXX and v3.5. Select the one with the highest version in Framework64/Framework directory depending on your requirement.
Copy the path of csc.exe and add it to the PATH system environment variable.
Quit the cmd, and then launch again and run the program. That’d work.