The root cause of this issue rests with the way mu-editor is currently packaged.
Upstream, mu-editor contains not just the editor but also uflash and the compiled MicroPython hex, so flashing the micro:bit works out of the box. In Debian, such bundling is not permitted, hence I have split out out uflash and MicroPython into their own packages. The placeholder firmware-microbit-micropython-dl package (provided by python3-uflash) sits in contrib, so cannot be added as a hard dependency on mu-editor. Once firmware-microbit-micropython is accepted into unstable (it is now in NEW) I can update the dependencies to ensure it is installed with the editor and avoid the issue you raise in this report. Thanks, Nick
