The following error appears when trying to add the IP addresses via "Home > Tools & Settings > IP Addresses" :
Error: Execute netsh interface ipv4 add address "Local Area Connection" 10.0.0.1 255.255.0.0 failed: The object already exists. (Error code 1)
IP was previously added to the server, but was not updated in IP addresses list in Plesk Control panel.
Click on "Reread IP" button in Plesk Control Panel->Tolls and Settings->IP Addresses button to make the added address visible.