With some providers they require the date of a leaver to be in the current period you're trying to submit, so you may come across across an error if it's in the period after, typically:
'Exit date must be less than or equal to pay reference end date'
It could be that you are sending the pension data for the calendar month of August, but the file says that the person in question is leaving on the 1st September. Some pension providers will reject this, as they do not allow "future-dated" notifications of people leaving the scheme. We see this fairly commonly on Smart Pension, for example. Typically the solution is to amend the exit date to an acceptable date, or remove it from this submission and send it in the next one instead, but this isn't always straightforward in payroll and you may be unable to change it if submitted to HMRC and completed P45s etc.
However, with Flexible File Correction you can simply amend the Exit Date to the last date of the period in your current submission. Once you have download the PAPDIS file by clicking on the 'Flexible File Correction' tab, you can change the date, save it and re-upload to PensionSync.
In this example, the period I am submitting is August but the leaver date is 1st September:
By Amending the ExitDate to 31/08/2021 your file will be accepted: