I also did find this BIG problem with ubuntu 18.04, some of the steps I did take are taken from various sources scattered all around forums, github, pullrequests etc, but at the end I succeeded.
At least on Ubuntu 18.04 when I found exactly the same problem with that very symbol missing,
You have to uninstall libnvtt and reinstall it from a very specific version, and be sure to setup the configuration properly. I did the following:
# unsure if this impacts other Linux versions however it seems you have to do that
apt -y install libssl1.0-dev
# uninstall libnvtt
apt remove libnvtt2 libnvtt-dev
# download good sources
git clone https://github.com/castano/nvidia-texture-tools
# change version and put 2.2.1
# change configure essentially
# AND enable shared instead of static (!) this was quite tricky to find out in the instructions for me (!)
# $CMAKE .. -DNVTT_SHARED=1 -DCMAKE_BUILD_TYPE=$build -DCMAKE_INSTALL_PREFIX=$prefix -G "
sudo ln -s /usr/local/lib/libnvcore.so /usr/lib/libnvcore.so
sudo ln -s /usr/local/lib/libnvimage.so /usr/lib/libnvimage.so
sudo ln -s /usr/local/lib/libnvmath.so /usr/lib/libnvmath.so
sudo ln -s /usr/local/lib/libnvtt.so /usr/lib/libnvtt.so
After this wizardry the assignment_client started working without that symbol error. Maybe it can work in the similar way on other systems.