When the grades are exported Moodle will export the ID Number if one is present in the user's profile. This is the "idnumber" field in their Moodle user profile.
When you import the same file after adding grades, you will have an option to map fields on the first page that appears after you select the file and begin the process. That is the "Import Preview" page.
If you select "ID number" in the "map from" dropdown box and that field is empty in the csv file you would see that error.
Moodle should not need a value for "ID number" because it allows you to select another field like the "email address" field to "map from."
For example you should be able to select "email address" from the "map from" field and then select "useremail" in the "map to" field.