When I tried to install a customized Android 10 to Pixel 3a, the following error occurred.
$ export ANDROID_PRODUCT_OUT=`pwd`; fastboot flashall -w
--------------------------------------------
Bootloader Version...: b4s4-0.1-5613380
Baseband Version.....: g670-00011-190411-B-5457439
Serial Number........: XXXXXXXXXX
--------------------------------------------
Checking 'product' OKAY [ 0.057s]
Setting current slot to 'a' OKAY [ 0.372s]
Sending 'boot_a' (65536 KB) OKAY [ 1.850s]
Writing 'boot_a' OKAY [ 0.349s]
Sending 'dtbo_a' (8192 KB) OKAY [ 0.320s]
Writing 'dtbo_a' OKAY [ 0.095s]
Sending 'vbmeta_a' (4 KB) OKAY [ 0.120s]
Writing 'vbmeta_a' OKAY [ 0.066s]
Rebooting into fastboot FAILED (remote: 'Unsupported reboot option reboot-fastboot')
fastboot: error: Command failed
I could solve this problem by installing a bootloader with the same build version and running flashall again. Corresponding bootloaders are included in factory images availabe in this page.