This probably means that you did not get the correct U-boot binary flashed as it is trying to execute the following,
But it wont be that easy and this probably requires some additional integration work. If i recall correctly, they have different U-boot configuration depending on where you will load it from, so you need to adjust this,
Also the sdimg can not have an embedded U-boot for eMMC so you would need to manually write it after you have flashed the sdimg, and this is the command according to Solidrun docs that you execute on a running system to flash U-boot,
echo 0 | sudo tee /sys/block/mmcblk0boot0/force_ro
sudo dd if=u-boot-spl-mmc.kwb of=/dev/mmcblk0boot0 conv=sync