This error message means that your study is completed since workers sometime submit the HIT to TurkPrime, but not to Amazon. This makes it that TurkPrime thinks that total amount of people already took your study, even though the TurkPrime progress bar is not yet full. However, the full number of requested participants should appear on your data file.
On occasion a worker may have completed a study but was unable to successfully submit the assignment to Amazon for a variety of reasons (timeout, system hiccup, computer shutdown). In such cases, the participants will usually reach out to the researchers and ask for a compensation HIT to be opened up for them.
Compensation HIT's can be set up by going into the Manage Workers page on the main menu bar and selecting "Create Compensation HIT".
Sadly, Amazon Mechanical Turk does not provide any other mechanism to pay workers who were unable to submit their assignments.
If this worker did not reach out to get paid, this likely means it was due to negligence on their end that they were not able to submit. You were not charged for these participants because TurkPrime only charges by the complete. If you'd like more workers to take the study, you can copy the panel and open it up to more workers while excluding the previous study from the current one.