vCenter server appliance authentication tab missing

I have configured our vCenter Server Appliance 5.1 with a second IP address so it can reach our active directory servers.

Now I want to add AD authentication to the environment, but the problem is when I open a browser and go to "https://IP_VCSA:5480", the authentication tab is missing.

Does anyone know how I can solve this problem? Or is there another way to configure AD authentication?

Thanks in advance!