By default, Artifactory disallows anonymous access to Git LFS repositories. This is defined in the Administration module under Security | Settings. For details please refer to Allow Anonymous Access.
The Git LFS client will ask for credentials for the Artifactory LFS repo when accessing it - if anonymous access is allowed you can just enter blank credentials, otherwise you should enter your Artifactory user name and password (not your Git one).
To make the authentication process automatic you can use Git Credential Helpers to store these for you and have the Git LFS client authenticate automatically.
Git stores credentials in plain text by default
You should take extra measures to secure your username and password when using Git credential helpers.