Why does my bootcamp assistant not show this option?

The reason for the Select Tasks page showing up versus what was shown in the YouTube tutorial you found is that your Mac does not support installing windows without creating a USB installer from the Windows ISO which is available on the 2015 and newer Macs. Basically you will need to select all three of the available tasks to continue and you will need an 8GB or larger flash drive. The following step will have you create the bootable flash drive and select the Windows ISO. After the USB drive is created then it will allow you to partition the internal drive for the Windows installation. For a detailed step by step please see Apple's support document here.