You’re running snapcraft on bionic (18.04) but building a snap based on xenial (16.04).
I’d recommend adding base: core18 near the top of your yaml and then run snapcraft. You should install multipass snap to build in. This will build based on 18.04 (core 18) and will get you past this error.
Onwards to more interesting errors later. This is a gtk3 app so you’ll need a gtk3 launcher for one.