Open Seven and go to > Settings located in the 'Workout' tab, then select 'Health' to check if the toggle is on. Then, go to the Health app in your device > Sources > Seven > turn all categories on and it should sync, but it will only show new data from after the date you activated it.
Also, please note that Seven does not sync walking or workouts under 7 Minutes. If you would like to log walking sessions that are longer than 7 minutes, you can go to your 'Profile' tab and tap the ‘Add Activity’ button. Here, you can log any of your external workouts that may not be logged through Health. Just make sure they are over 7 minutes!