In Microsoft Office SharePoint Server 2007, a trusted file location is a SharePoint document library, a UNC path, or an HTTP Web site that is configured as a trusted repository for workbooks that Excel Calculation Services can access. Excel Calculation Services opens workbooks that are stored in trusted file locations only.
Simple solution when I troubleshoot this issue “You do not have permissions to open this file on Excel Services.” on Excel Web Access.
In the Trusted File Location, the site is added, but the Trust Children is uncheck. Check on the Trust Children to allow any document library within the site enabled as trusted file location.