bloom I haven't tried it myself, but it seems like the instructions from the readme are out of date? Apparently you should have installed the following:
flatpak install flathub org.freedesktop.Platform//18.08
flatpak install flathub org.freedesktop.Sdk//18.08
But your error message mentions missing version 19.08
. So I would try running the above two lines again with the 18.08
mentions changed to 19.08
.