This looks like you’ve tried to create a custom Java Action called Cell_WriteByIndex, but you’ve not added any Java code into the action itself.
The default behaviour of a new Java action is to throw an exception saying “Java action was not implemented”.
To fix this, you will need to open the Java action in Eclipse or your favourite IDE / Text Editor and add your custom code.
https://docs.mendix.com/howto/logic-business-rules/extending-your-application-with-custom-java/
I hope this helps.