SharePoint Excel Service : How to resolve "the workbook cannot be opened" error?

Run following commands for each web application that is associated with Excel Services:

$w = Get-SPWebApplication -identity http://<WebApplication>
$w.GrantAccessToProcessIdentity("<Domain>\<Username>")


In future, for the additional content databases, you must rerun these cmdlets to ensure that Excel Services has access to the new databases.


Source: https://docs.microsoft.com/en-us/sharepoint/administration/configure-excel-services