Researchers are supposed to provide these codes, but sometimes they forget. The study may also be designed to register your submission automatically as complete upon finishing the survey, and you will brought back to the Prolific website.
If there was a problem with the completion code or URL for your study, this should not cause a problem that prevents you from getting paid as long as you've made a valid submission.
If you have not yet completed the submission by clicking "Complete" or "I didn't receive a completion code" do this.
Next check the status of your submission, it should read "Awaiting Review"
If you have the completion code "NOCODE" this doesn't mean your submissions is invalid, just that the researcher may need to review it before approving. This won't affect your Prolific Score in any way and you can assume that you will receive the reward (if you have completed the study diligently).
You may want to contact the researcher to let them know there was a problem and that they may need to review your submission.
If you do the above and still do not see the study you just completed in your submissions, please contact the support team.
Editing Completion Codes - You can update completion codes by going to 'Submissions', clicking on the completion code and editing it.