It is indeed not much you could have done wrong.
You can check the output of ls -l (list directory) from within the directory you named above and started bash .flash.sh from. If you are unsure if all is correct post a screenshot of the output here.
But you can see the folders content in any filemanager so terminal is not even necessary.

This really comes down to the simple question if the sony image is in place and named correctly.
There is a space character before .img in the filename you posted above. If that is present in the filename on your computer it would be wrong. But i guess the space is just a wrong representation here on TMO since it is missing when i qoute your post.

If all is correct i would suggest to open a question at together.jolla.com since actual Jolla engineers are reading and answering.
I have heard of Android Platform version related errors on macos but those where more obvious and fixed in 2.0.1.4