For some reason, I am unable to partition my SSD.
Disk Utility reports a small amount of disk space, but this is a known issue to do with local snapshots:
Note: You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed.
I actually have plenty of free disk space: