By default VNC Connect only allows users who are members of the Administrators group to connect.
To allow non-administrator accounts to connect, you will need to add the user or group into VNC Server's Users & Permissions options.
You can either add a group of pre-registered users or individual accounts. Users can be registered from both local and domain accounts and groups.
When running VNC Server on Windows, users and groups can be selected using the Object Picker UI:
Once the user has been added, permissions can be set for that user/group and users should now be able to authenticate successfully.