I've had the pesky notification for the locked folder on my phone since August, I FINALLY found a fix. I forgot my password and haven't been able to make it go away... but I found a way...read below
1. Go to Secure Folder
2. Enter any "password", it will be wrong of course
3. It will notify you it's incorrect and below after you enter this wrong password will pop up the words "forgot password" -click it
4. It will prompt you to login to your Samsung account for security reasons. Log in. If you also forgot that password you can also reset that on this screen.
5. Once you log in to the Samsung account, it will allow you to change your secure folder access/password on this next screen.
6. Once you update the password, u can access the folder and notification will disappear.