You need to sign in to do that
Don't have an account?
Unable to complete Trailhead Challenge Build a Battle Station App : Adding Business Logic
Hi, I am unable to complete the module Adding Business Logic, where I am getting the below error message. Please help.

Then I would suggest you log out of all the accounts and log in again and also clear your browser cookies and history.
OR
Another solution would try it in a different browser.
Please feel free to contact me if you need any further information.
Thanks
RahulKumar
I tried the steps you mentioned but its not working.
If you've completed the challenge correctly, but you are still getting errors, make sure that you are using a fresh Developer Edition that does not have a namespace prefix.
Please check the below link which had similar kind of issue.
http://developer.salesforce.com/forums/ForumsMain?id=9060G000000I2IkQAK#!/feedtype=SINGLE_QUESTION_SEARCH_RESULT&id=906F0000000kBbyIAE
Let us know if this will help you.
Best Regards
Rahulkumar