This can be easily fixed. You need to create a trust location for your document library.
Please refer this- http://sharepointknowledgebase.blogspot.com/2011/12/this-workbook-cannot-be-opened-because.html
As the link said: How to add a file to trusted file location:
On the Start menu, click All Programs.
Point to Microsoft SharePoint 2010 Products, and then click SharePoint 2010 Central Administration.
On the SharePoint 2010 Central Administration page, under Application Management, click Manage service applications.
On the Manage Service Application page, click Excel Services Application.
On the Manage Excel Services Application page, click Trusted File Locations.
On the Excel Services Application Trusted File Locations page, click Add Trusted File Location.
On the Excel Services Application Add Trusted File Location page, in the Address text box, type the location to save your workbook
Under Location type, click the appropriate location type. In this example, select SharePoint Foundation.
Under Trust Children, select Children trusted if you want to trust child libraries or directories.
Click OK.