Difficult to see what is the actual problem, but the error states that the PlainBody variable is declared as final in the java code and the action is trying to change the value which is not allowed.
To fix this error open the project in Eclipse and open the specific class, you will see there error in the code that needs to be fixed.
Hi Gowtham S,
Could you try updating the Marketplace Module of Email Connector in your project?
A lot of the times the java actions errors of this kind are resolved by updating the modules from the error messages that you get.
I hope that helps you with your issue, best regards!