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 and tap your profile icon > Apps > Seven > Turn all categories on. Please note that this will only sync new data from after the date you activated it, not past workouts.
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 'Workout' tab and tap the ‘Add Activity’ button at the bottom of the screen. Here, you can log any of your external workouts that may not be logged through Health. Just make sure they are over 7 minutes!