When uploading schedules to SmartPM the data date / status date is required. The status date is the date for which the information in your schedule is accurate of.
This error message is specific to Microsoft Project only because P6 requires the data date to be set, whereas Microsoft Project does not. When importing schedules to SmartPM an error message may pop up that your schedule is missing a data date (also known as status date in MPP files). First recommendation is to talk to the scheduler to know for sure it is the accurate date to set. You are also allowed to manually pick the status date in SmartPM with the calendar menu within window. A deeper understanding can be found under Help tab, where you can search for "Prepping Microsoft Project for Import". This video will also walk you through where to find the status date within your Microsoft Project.