A correct completion code shows that the participant clicked on your completion url.
e.g. going to
will complete the study with the code 12345.
If a participant completes the study on Prolific without entering the code or clicking on your completion url you will see NOCODE where the completion code should be.
This doesn't mean that the participant didn't do your study.
The participant may have just missed your completion url by mistake. However, if you see NOCODE you may want to confirm that the participant did your study correctly before approving their submission - read more about this here: http://help.prolific.ac/managing-and-reviewing-submissions/dealing-with-issues/problems-with-the-completion-url-nocode-submissions