Please refer to my procedure.
1. Please modify configs/ubuntu/additional_packages_list as the following.
additional_lite_packages_list="systemd udev net-tools iputils-ping isc-dhcp-client libxml2-dev libnl-genl-3-dev libreadline-dev libtinfo-dev sudo"
2. Please remove build/rfs/rootfs_lsdk1909_LS_arm64_lite/
$ flex-builder -i clean-rfs -r ubuntu:lite
or
$ sudo rm -rf build/rfs/rootfs_lsdk1909_LS_arm64_lite/
3. Please rebuild unbuntu:lite rootfs
$ flex-builder -i mkrfs -r ubuntu:lite -a arm64
4. flex-builder -c linux -a arm64 -m ls1028ardb
5. $ flex-builder -c apps -r ubuntu:lite -a arm64
6. flex-builder -i merge-component -a arm64 -r ubuntu:lite
7 . After execute the following command, please deploy build/images/rootfs_lsdk1909_LS_arm64_lite.tgz to your target board.
$ flex-builder -i packrfs -r ubuntu:lite
