sudo apt-get qttools5-dev and/or qttools-dev-tools should take care of that. I'm afraid I've lost track of which version of which os needs which.
Looks like bionic is at sigil 0.9.9 in its repos. If they make it to 9.9.10 soon you may want to consider sticking to the repo version. That way system upgrades won't break it.
I just had a similar thing happen to me on Arch. The upgrade of the system Python to 3.7 broke my custom compiled version of Sigil that was built against 3.6.
EDIT: Make sure to check the Linux build docs in the src directory
or online. The Qt5 dependency list was recently updated based on
your exact issue, it seems. qttools5-dev seems to be the missing culprit on newer Ubuntu versions.