Whole drive gets wiped when attempting to re-format as NTFS for Windows
At the point where you received the error message, you should have seen a partition labeled BOOTCAMP. You need to select this partition and click on the Format button. The label BOOTCAMP will disappear. Next, you select the partition that was labeled BOOTCAMP and proceed with the installation of Windows.

The Boot Camp Assistant cannot NTFS format a partition. So, instead the partition is formatted FAT32 with the label BOOTCAMP. You have to click on the Format button to reformat the partition as NTFS. This will erase the BOOTCAMP label. Later, when the Windows Support Software from Apple executes, the NTFS-formatted partition will be renamed back to BOOTCAMP.