Steam says I do not have enough disk space available to install a game, even though I have plenty. How do I solve this?
Solution 1:
Steam probably displays in mebibytes (1024 kibibytes per mebibyte) and your file manager in megabytes (1000 kilobytes per megabyte).
Converting 12846MiBs to MBs give us 13470MBs, which is just a bit less than the space you have available, but with cache usage and stuff like that, the storage you have free might fluctuate, causing steam to think that you can't install the game.
The solution is to clear up your drive a bit (and as you're on manjaro, try out pacman -Sc, which will allow you to clear old versions of the packages you have installed).