In order to install AION, you must have at least 40 GB free on your hard drive. From what I know, the game needs about 14 GB to install After that, updates will start downloading on your computer. Let's say you have 33 GB free on your hard drive. Being an online game, means that it will constantly need free space. Although from your calculations, space is provided, the Installer finds your amount of space left on the hard drive as insufficient. Try deleting, or if you don't want to lose files, try moving some files from a partition to another, and make sure you have a minimum of 40 GB free space before installing the game. That should be more than enough, even for further updates.
The error clearly says you don’t have enough free space on your hard drive to install the game. If you don’t know how much free space is required for the game, check the system requirements. Aside from the required hard drive space, you also need to make sure your computer meets the rest of the requirements. Aion Online supports Windows 7 and Windows 8.x both 32-bit and 64-bit.
It requires 2.8 GHz Intel Pentium 4 or equivalent AMD Sempron+ 2800 with SSE2 support or faster processor; 1 GB RAM; 40 GB free space; and NVIDIA GeForce 6800 GT, ATI Radeon X800 XL series, or better video card. Now, since the required free space is around 40 GB, the free space on your system drive or drive C must be higher than this.
Since the game installs on the drive where the operating system is installed by default like any other program, the free space must be ten to twenty GB (10 – 20 GB) more than the game’s free space requirement to make sure the operating system will still run properly.
So, before you install the game, drive C must have at least fifty to sixty GB (50 – 60 GB) of free space. If you don’t have this available space, try to uninstall some of the programs you don’t use to free up space. For those who want to try the game, head down to Aion Online homepage and download the client.