The available space shown will show 0MB if the entire drive you choose has been partitioned into 1 partition. This means that your entire partition takes up all of the drive space.

With this type of partition setup, there is no "non-partitioned" space available for the installer to create a new partition. It does not matter if you have free "formatted" space. The installer looks for free "unformatted" and "unpartitioned" space.

So you need to make sure to choose the option to "shrink existing partition (non-destructive). this will allow the installer to change the size of your existing partition and create 400MB for the Traktor FS installation.