Try running
$ sudo apt-get install gawk wget git-core diffstat unzip texinfo gcc-multilib \ build-essential chrpath socat libsdl1.2-dev xterm
directions from:
https://www.yoctoproject.org/docs/2.0/yocto-project-qs/yocto-project-qs.html
I just successfully built the rootfs for CycloneV a couple of days ago on Ubuntu 18.04.