There may be a Standard report in Cognos, but if not you could pretty easily build the report with either Query or Report Studio. To get it into Excel, you would just need to run the Cognos report and choose Excel as the output format.
I didn't see a standard report for this (though there is one to see whether approvers have viewed receipts or not), but there are definitely data items in reporting you can use to get what you need.
Under Expense reports, Report Header Information, and then Receipt Information, there's "Receipts Received" for receipts attached at the header level. Under Expense Reports, Entry Information, and then Receipt Information, there's also a "Receipts Received" for receipts on the entry level. Both have a "receipts required" item as well, so you can differentiate whether something should have been attached or not. You'd have to rename columns as you go or multiple columns will end up looking the same between the entry line and report header sources.
You'd have to add the employee (and group it or sort by it so your exported report will be by person), report name or ID, and possibly something to separate out each entry of a report (expense type is likely easiest) so the information will show correctly. Then you could filter down as you needed to find the reports that had no receipts attached.
This isn't exactly a step by step, but I hope it's helpful!