Error Code: 10002 Merchant Authentication failed.
Don't worry... It happens to the best of us.
This problem come when you are using the old Panel of CCAvenue
Call CCavenue Customer Support ask for new panel login and password they will require some information to verify your account from your registered email ID, then they will send your the new login and password to your registered email id.
Login to new Panel go to settings >> API Keys you will have all your new API keys.
Now go to Resources >> Web Instigation Kit and download new instigation Kit.
Do all the changes as required.
This will fix the error