> Steam and free space
Although macOS is meant to free up “purgeable” space when it’s needed, installers such as Steam’s don’t always work properly. Even if “Optimize Mac Storage” is enabled, some installers fail to get macOS to delete local copies of files in iCloud Drive.
From Time Machine’s menu bar, choose Back Up Now. Next, go to Apple Menu > About This Mac > Storage to check free space; backing up should have purged local snapshots — which, in High Sierra, are created on every APFS-formatted, all-flash storage device connected to your Mac.
Space still not free? In Terminal, use the mkfile command to create a file as big as the space you need. So, mkfile 30g testfile.dat will create a 30GB file. Then trash the file and empty the Trash; use this to free up space used by local copies of iCloud Drive files, making it available to problematic installers, which then see it as available, and keeping your cloud-based copies of files.