Why do we need to enable force authentication?
When working with NuGet repositories and the "Anonymous Access" option is enable, we set the permissions to the Nuget repository, therefore we need to enable the "Force Authentication" option at the repository configuration. When this option will be enabled Artifactory will ask the client for credentials and will allow the download according to the permissions.
After you will set the "Force Authentication" option kindly add the username and the password to the NuGet client configuration, you can add the credentials using the following command:
$ nuget sources update -Name <source name> -UserName <your user name> -Password <your password> , for example:
$ nuget sources update -Name Artifactory -UserName admin -Password password
You may want to read more about "Anonymous Access to NuGet Repositories" in the following wiki page.