PDFsam is developed using Java 8 and JavaFX. It should work on higher
versions provided they come with their JavaFX, as far as I know Oracle JDK
still bundles JavaFX, with a plan to separate the two soon. OpenJDK already
separated JavaFX available through the package OpenJFX but from what I can
see, the package in the repo https://packages.debian.org/sid/openjfx is for
OpenJDK 8. What happens is that PDFsam starts with a higher version of
OpenJDK (on my brand new box is OpenJDK 11) for which there is no OpenJFX.
A quick fix would be to force the dependencies to openjdk-8-jre + openjfx
