Briefcase uses an Ubuntu 18.04 base Docker image (we used 16.04 in the past, but that was updated over a year ago).

From my testing, it appears to be an issue with PySide6 itself. I've got a an Ubuntu 18.04 test machine, and I get the same GlibC error - which would be consistent with the fact that Ubuntu 18.04 ships with GlibC 2.27.

Based on this post, it would appear that Qt may not support Ubuntu 18.04 - however, I'm not sufficiently familiar with Qt to be able to say that canonically. I suspect this may be an issue you need to take up with Qt support channels.

If Qt doesn't support 18.04, then you'll need to develop a custom deployment for a different base image. To do this, you'll need to:

1. Compile the Python-linux-support project on a more recent Linux (it looks like Ubuntu 20.04 should work), generating a support package tarball.

2. Add a support_package instruction to your pyproject.toml that points at that support tarball

3. Fork the briefcase-linux-appimage-template and update the Dockerfile to use the same base image version that you used to compile the support package

4. Add a template instruction to your pyproject.tomml that points at your template fork.

That should then allow you to build an AppImage that is based on a Linux distro that has Glibc 2.28, and will thus support Pyside6.

At some point in the future, Briefcase will likely do this upgrade in an official capacity - but I can't say when that will happen.

In the meantime, I've logged #692 to track the problem.