After I ran the command you gave me, I check the Disk Utility - Partition Option - and then it says "Free Space", so I'm pretty sure the command worked, but I'll still give you the output of diskutil list:
(When I ran the command to make Free Space)
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI 209.7 MB disk0s1
2: Apple_APFS Container disk1 750.0 GB disk0s2
/dev/disk1 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +750.0 GB disk1
Physical Store disk0s2
1: APFS Volume M5 566.7 GB disk1s1
2: APFS Volume Preboot 42.0 MB disk1s2
3: APFS Volume Recovery 517.0 MB disk1s3
4: APFS Volume VM 4.3 GB disk1s4
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *15.5 GB disk2
1: DOS_FAT_32 WININSTALL 15.5 GB disk2s1
(After 2nd try for Windows Install)
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk0
1: EFI 209.7 MB disk0s1
2: Apple_APFS Container disk1 750.0 GB disk0s2
3: Microsoft Reserved 16.8 MB disk0s3
4: Microsoft Basic Data 250.0 GB disk0s4
/dev/disk1 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +750.0 GB disk1
Physical Store disk0s2
1: APFS Volume M5 565.0 GB disk1s1
2: APFS Volume Preboot 42.0 MB disk1s2
3: APFS Volume Recovery 517.0 MB disk1s3
4: APFS Volume VM 4.3 GB disk1s4
/dev/disk2 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *15.5 GB disk2
1: DOS_FAT_32 WININSTALL 15.5 GB disk2s1