Microsoft Office Excel cannot access the file ‘c:\inetpub\wwwroot\Timesheet\App_Data\Template.xlsx’

Try this:

1. Create the directory

C:\Windows\SysWOW64\config\systemprofile\Desktop

(for the 32-bit version of Excel/Office on a 64-bit Windows computer) or

C:\Windows\System32\config\systemprofile\Desktop

(for a 32-bit version of Office on a 32-bit Windows computer or a 64-bit version of Office on a 64-bit Windows computer).

2. For the Desktop directory, add Full control permissions for the
   relevant user (for example in Win7 & IIS 7 & DefaultAppPool set permissions for user IIS AppPool\DefaultAppPool).

Original post with answer:

• Excel 2007 automation on top of a Windows Server 2008 x64