As you can see, the second partition of SD card is empty, so you will get the error when booting.
Please try to execute the following command on your PC to deploy image to SD card.
$ flex-installer -i pf -d <device>
$ flex-installer -b boot_LS_arm64_lts_5.10.tgz -r rootfs_lsdk2108_ubuntu_main_arm64.tgz -d /dev/sdx
In the above command, the SD storage drive in the Linux PC is detected as /dev/sdX, where X is a letter such as a, b, c. Make sure to choose the correct device name.
If your problem persists, would you please provide your console log running "flex-installer" command?