When opening an excel file I get the following error "This workbook cannot be opened because it is not stored in an Excel Services Application trusted location.

If you are working on Office Web apps in SharePoint 2010 then you may come across this error…

“This workbook cannot be opened because it is not stored in an Excel Services Application trusted location.

To create an Excel Services Application trusted location, contact your system administrator.”

its very simple to resolve this error and start working on excel again :-)

there are 2 methods to resolve this issue:

Option 1: Configure to open in Client Application

At the site collection level activate the "Open Documents in Client Applications by default Feature"

Option2: At the Document Library set the "Opening Documents in the Browser" setting to "Open in client application"

OR

1. open central admin 
2. go to Manage service applications
3. Click on Excel services Application.
4. Then click on “Trusted file locations”
5. You can see that http:// is already there, so you could just add https:// or put in a url to a specific location
6. Fill in the boxes, I also ticked the trust children box.
7. Once you have done this go back to your Excel document and it will open up in the browser. 

If you have any queries/questions regarding the above mentioned information then please let me know, thank you