It seems you’re trying to access the document approval field from the Project (parent) object, which isn’t directly available and that is the reason you're getting nothing / blank.
The easier approach is to create a Document Approval Report, where the Approval Date field is by default and you can then filter it for documents containing “Quote”.
Alternatively, you can capture this value using Fusion and store it in a custom field on the Project, so it’s visible in your Project Report.
