Im attempting to use Apple's Boot Camp Assistant. When I select Continue it displays "The disk does not have enough space to be successfully partitioned. Your disk can only create a 24 GB partition. The Boot Camp partition requires at least 42 GB."

Does it say "Available" or "Free"? You must have at least 42GB of Free storage space since that is the only storage that can be used immediately. "Available" space may not be immediately usable, but will be released for use at some unknown time in the future.

The only places where macOS specifically mentions "Free" space is in Disk Utility or in the Storage section in the Apple System Profiler. Otherwise you will need to calculate the Free space using this formula:

Free space = Available - Purgeable

FYI, I highly recommend you have much more than 42GB of Free storage space since macOS requires at least 20GB+ of Free space at all times......even more space depending on the workload. If your macOS boot drive is only a 120GB SSD, then you do not have a large enough drive to dual boot.....a 120GB drive can barely run macOS by itself.