ClientContext Does not Contain a Definition for AuthenticationMode

Question

I had a script that worked earlier today, but when I restarted my machine I now get ClientContext does not contain an definition for 'AuthenticationMode'. I have tried reinstalling the Nuget Packages. I have tried restarting my machine, no luck. I have another project that uses the same clientContext.AuthenticationMode, and also works fine. How do I fix this reference issue?

ClientContext.Credentials works but ClientContex.AuthenticationMode does not.

    public void GetSharePointSite()
    {
        string sharepointURL = $@"https://mySharepointSite";

        using (ClientContext clientContext = new ClientContext(sharepointURL))
        {
            clientContext.AuthenticationMode = ClientAuthenticationMode.Default;
            clientContext.Credentials = CredentialCache.DefaultNetworkCredentials;

            //load subsites and their titles 
            Web web = clientContext.Web;
            clientContext.Load(web);
            clientContext.ExecuteQuery();

            Console.WriteLine(Convert.ToString(web.Title));
        }
    }
}