How do I delete purgeable space detected by Disk Utility?
See Traderhut Games' answer from this answer:

In Terminal, run

df -h /

Filesystem      Size   Used  Avail Capacity iused       ifree %iused  Mounted on
/dev/disk1s1   466Gi  370Gi   64Gi    86%    xxxxx   xxxxxxxxx    0%        /


then, for the disk you wish to use, run:

diskutil secureErase freespace 0 /dev/disk1s1