dongle-flash not working

$ dongle-flash loopback.hex
packaging iHex using nrfutil ...
Error: No such file or directory (os error 2)

this indicates that nrfutil, the Python tool, is not installed or not available in your PATH. Instructions on how to install nrfutil can be found in the setup page and in the top-level README of this repository. If you install nrfutil in a virtual environment you'll need to activate the environment; the nrfutil binary must be available in your PATH.