Unable to flash eMMC

I’m following the instructions at Install OS on eMMC | Radxa Docs to try and flash Debian onto my Zero 3W 8GB with 64GB eMMC, but I get an error in the RKDevTool (see attached screenshot). What am I doing wrong?

this is the first time you flash the image on emmc? can the board boot from sdcard?

I was able to get it working by building and running rkdeveloptool on my mac. Not sure what was wrong with the RKDevTool GUI.