If you receive this error message, it means your study is completed but because of a glitch in the system at least one worker was unable to submit their HIT to MTurk.
If you receive this error message your study is complete. However, because workers are sometimes able to submit their HIT to CloudResearch but not MTurk, the CloudResearch system correctly believes the total number of requested participants have already taken your study even though the progress bar is not yet full. If you check your data file, the full number of requested participants should appear there.
Dealing with the Discrepancy
When you receive the error message above, at least one worker submitted their study to CloudResearch but the information was not relayed to MTurk. In such cases, the worker will usually reach out to you to ask for compensation. To compensate the worker, you can open a Compensation HIT.