Got the below error, while updating the Merged plugin dll in SIT environment.
Started comparing the differences between Dev and SIT environments by navigating to
Microsoft Dynamics CRM -> Settings -> Customization -> Customize the System -> Plug-in Assemblies.
Clicked on the required Plug-in Assembly and started comparing the number of plug-in class files in it.
Found that, in the SIT environment have extra plug-in class files, which were imported in the beginning of the project and which were not present now in the updated Merged Plug-in Assembly Dll (Removed as per client requirement from the Dev code).
Finally, deleted those files manually in the SIT environment by connecting to the Plugin Registration tool and unregistered them.
Updated the required plugin assembly in SIT. It worked and issue resolved :):):)