Step 1. Connect your Samsung Health to Health Connect
Open your Samsung Health app
Go to Settings > Health Connect
Grant permissions to share your Samsung Health data with Health Connect
Reference
Turn on Sync with Samsung account . tap Sync now
Reference
Step 2. Grant permissions of Health Connect to Lifestack
Go to Settings > Apps > Lifestack
Click Permissions
Make sure to grant permissions for Health Connect
(After you confirm that the sleep record appears in Samsung Health, you should also see that Lifestack has accessed it in the Recent Access section in Health Connect)