The Steptember app manually syncs with Google Health Connect (formally Google Fit) (Android users) or Apple Health (iOS users). So first, make sure you connect your wearable e.g. Fitbit with Google Health Connect or Garmin with Apple Health.
Then, in your Steptember app account, select 'LOG STEPS' and choose 'Sync from Apple Health' or 'Sync from Google Health Connect'.
If you are recording your steps or activity any other way, you will need to add your daily steps and activity manually.
If you are having syncing issue please check your health app permissions.
Please check your app permissions are turned on, and if not, select 'Turn On All':
Please see How do I log steps using the app?