People's Pension error:
H/P04 - File start date cannot be later than YYYY-MM-DD
People's Pension has a check that the first enrolments/contributions are within 3 months (plus a few days) of the entered Staging Date.
This is not correct validation, as there are valid reasons why this would not be the case - a change of provider or no Eligible jobholders 3 months from Staging are typical examples.
To solve, please contact People's support and ask them to amend the first allowable period to be the period you are trying to submit. Once amended by People's you can submit the data again.