...
When you select Rootfs as "Raspberry Pi(2020-08-20-raspios-buster-armhf)", the "Boot from" item will fixed to SDCard. Please burn the image file to SDCard for bootup.
Use Win32DiskImager, BalenaEther or other SD card writers to write the unzip Raspbian image to an SD card. Note that size of SD card should be greater than 8 GB. Refer to step 4 of How to build SP7021 Linux image for booting from SD card for how to write image file to an SD card.
Figure 5 shows the picture of Yocto Image Maker page after run finished. You can see the 2020-08-20-raspios-buster-armhf.img.bz2 word with blue color in the main window. Click 2020-08-20-raspios-buster-armhf.img.bz2 word to download the bin file.
...