Then you have to install multiarch-support package from 18.04 LTS first with

cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/main/g/glibc/multiarch-support_2.27-3ubuntu1_amd64.deb
sudo apt-get install ./multiarch-support_2.27-3ubuntu1_amd64.deb

and continue to follow the guide.

Why are you installing Kali Linux but switching away from the Kali repositories to Ubuntu? The two might not be entirely compatible. I do hope that you have not already updated some packages and installed the wrong ones.

See the article Fix Kali Linux sources.list Repositories. The only repositories needed in sources.list are:

deb http://http.kali.org/kali kali main non-free contrib
deb http://security.kali.org/kali-security kali/updates main contrib non-free

Don't forget to run sudo apt-get update.

I don't know why you are making the effort of installing multiarch, but if it's for installing another package, try now to install that package first without multiarch.

You could also install multiarch-support manually:

cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/main/g/glibc/multiarch-support_2.27-3ubuntu1.4_amd64.deb
sudo apt-get install ./multiarch-support_2.27-3ubuntu1.4_amd64.deb

libfltk1.3-dev doesn't support co-installation of multi-arch variants (yet?), so you can't install both amd64 and i386 flavours simultaneously to build 32- and 64-bit programs at the same time. (For libfltk1.3-dev, this is because the package includes fltk-config; to allow multi-arch co-installation would require pulling that out into a separate package.)

Currently the most versatile approach is to use chroots; if you're building Debian packages, then sbuild supports multiple architectures out of the box, otherwise you'll need to use debootstrap manually.