The error seems to be quite simplistic, but you might need to do quite a bit to resolve this. I created a new BI site and was welcomed by the Error Message "The WorkBook cannot be opened". Did some google and tried to find out a solution. Found a good post which kinda came to the rescue to resolve this irritant.
You can follow the article by Jeremy to resolve the issue:
http://blogs.msdn.com/b/jjameson/archive/2010/05/04/the-workbook-cannot-be-opened-error-with-sharepoint-server-2010-and-tfs-2010.aspx
I have do extra apart from what has been already mentioned in the article:
1. Start with the EventViewer. Thats the right place to get to know any errors pertaining to this issue
2. I had multiple accounts hence had to give access rights to all of them the db_owner access on the WSS_Content_<Guid> database
3. Again you can refer to event viewer and it gives the name of account which requires access
4. After resolving the Access issue , I got the compatibility mismatch error in the event viewer
There is a compatibility range mismatch between the Web server and database
"WSS_Content", and connections to the data have been blocked to due to this
incompatibility. This can happen when a content database has not been upgraded
to be within the compatibility range of the Web server, or if the database has
been upgraded to a higher level than the web server. The Web server and the
database must be upgraded to the same version and build level to return to
compatibility range
5. You would need provide the Process Identity Access in order to resolve this issue.
6. Step 5 needs to be done for all the accounts you are using, in my case I was using two accounts
7. Finally, doing all this I still faced the same issue.
8 ***IMPORTANT*** - Do a iisreset and Sart a New Session in Browser for the changes to come in affect.
Hope this helps.
Cheers
Aman Garg