Fix "This workbook cannot be opened because it is not stored in an excel services application trusted location" Error in SharePoint 2010

Problem: Got this error when trying to open a Excel Sheet stored in a SharePoint 2010 document library: "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"

Solution: By default, Excel Services allows browser rendering on Workbooks that are in "Trusted location". Add your SharePoint library/site/webapp URL to Excel services trusted file location to get rid of this issue. Here is how to create excel services trusted location:

• Navigate to SharePoint 2010 Central Administration site
• Under Application Management, Click on Manage Service applications
• Select "Excel Services Application" and click on "Manage" 
• Click on "Trusted File Locations" and then Click on "Add Trusted File Location"
  
  
• Enter the URL of your SharePoint web app/site/library and choose "Trust Children"
  
• Click "OK" to save your configuration changes.

Now, your Excel sheet should render without any issue.

Technet Reference: Manage Excel Services trusted location - https://technet.microsoft.com/en-us/library/ff191194.aspx. Its also possible to manage trusted locations using PowerShell. Here is the reference: Excel Services cmdlets