I'm not sure why it's not already pre-populated, but you need to put the IP address of your internet gateway. In most home setups, that will usually be your router, depending on your internet connection, it may also be incorporated into some sort of modem. Or it may be a hardware firewall in a more advanced setup.

Also you need to make sure that the static IP you set is on the same subnet as the rest of your LAN. Otherwise you won't be able to connect to it. These days, the most common one in consumer grade gear is 192.168.1.0/24 which will give you the IP addresses 192.168.1.1 through to 192.168.1.254. Regardless, if you can find out the IP address of one of your LAN connected devices, if you give your server an unused static IP with the same first 3 sets of numbers (e.g. 192.168.1.x, or 192.168.0.x, or 10.0.0.x, etc) then you should be good.

One other thing that may cause issues is if you have an IP clash. That shouldn't ever happen when you use DHCP. But it's easily done when you set a static IP. I recommend that you try pinging the IP address without your server off or disconnected to see if anything else is using it. That's not always 100% reliable (as some devices may be configured to not respond to ping) but its a quick and dirty test that can be a bit of a guide.

Finally, TBH, installing TurnKey to bare metal on a machine like that seems like such a waste of resources! Personally, I'd install a hypervisor like ProxmoxVE. You can download TurnKey LXC containers direct via the Proxmox UI (if you haven't already heard, containers are an awesome way to run Linux OS, giving near bare metal performance - relative to the resources you allocate to the container). Plus you can also install from ISO if you want a full VM (or other OS such as Windows).

FWIW I have Proxmox running on a low power Intel Atom based server (glorified NAS really) and currently I have about 5 containers and 2 VMs running. I mainly use it as something of a home lab, for testing etc. It's a good thing!