Hi Anuj,
10.8 Desktop .Net SDK is based on different .Net framework version (4.5.2). ArcGIS 10.2 based on 3.5.1.
Calling application (ArcMap) .NET version must be greater or equal tool .NET version.
Your ArcGIS Destop and ArcObjects SDK version must match. You can build tool on 10.2 and run it on 10.8 (with some changes to ArcMap config file), but not opposite.