Make sure that you have Office runtime installed on the server.
If you are using Windows Server 2008 then using office interops is a lenghty configuration and here are the steps.
Better is to move to Open XML or you can configure as below
Install MS Office Pro Latest
Create User ExcelUser. Assign WordUser with Admin Group
Go to Computer -> Manage
Add User with below options
User Options Password Never Expires
Password Cannot Be Change
Com+ Configuration
Go to Control Panel - > Administrator -> Component Services -> DCOM Config
Open Microsoft Word 97 - 2003 Properties
General -> Authentication Level : None
Security -> Customize all 3 permissions to allow everyone
Identity -> This User -> Use ExcelUser /password
Launch the Excel App to make sure everything is fine
3.Change the security settings of Microsoft Excel Application in DCOM Config.
Controlpanel --> Administrative tools-->Component Services -->computers --> myComputer -->DCOM Config --> Microsoft Excel Application.
Right click to get properties dialog. Go to Security tab and customize permissions
See the posts here:
Error while creating Excel object , Excel manipulations in WCF using COM