Smart Viewer has its own local user database (independent from the NVR users) which requires you to setup a unique password for the ADMIN user account the first time the application is launched.
If the user forgot the ADMIN password - there is no password reset option.
This is designed on purpose to prevent unauthorized access to the admin user account.
How can this be fixed?
Smart Viewer needs to be uninstalled completely, this will delete the user database and will also delete "all settings and configuration in Smart Viewer".
Download Smart Viewer from this link and re-install: https://www.hanwhasecurity.com/smart-viewer.html