If your computer uses '32-bit architecture' you need 16Gb of free space. If it's '64-bit' you require 20Gb. So you're well short of what's required.
Further, as your machine seems to be quite old, I'd be worried about whether it's got enough RAM to run Windows 10. You need 1Gb for 32-bit systems and 2Gb for 64-bit systems.
You can check whether you've got a 32- or 64-bit system, and how much RAM you've got, by going to Start and thenquote[right]-clicking on Computer and selecting 'Properties'.
However, if your computer is correctly reporting the size of its hard drive, I doubt that you'll be able to run Windows 10 on it. You could possibly try inserting a high-capacity memory stick into a USB port (see under 'Important Notes' here:
https://www.microsoft.com/en-gb/windows/windows-10-specifications ) but I can't really see that working very well, if at all.
Unless you install a second hard drive (and can then persuade the Windows 10 installation process to use that drive as your boot drive, instead of drive C), I doubt that you'll be able to run Windows 10 on your computer. (Don't worry though; simply join the millions of people, including me, who're sticking with Windows 7 or 8.1 anyway!)