ERROR: opening 'weather.rrd': Aucun fichier ou dossier de ce type

You need to issue this command from that directory where weather.rrd is located. Should be /usr/local/share/weatherradio or /usr/share/weatherradio.

I also tried to try to decipher the data file, but I think there will be a problem, because the cloud cover depends on a calculation from the sky temperature, as the SQM depends on the Lux value.
So how to do it?

Cloud coverage is stored directly, so you can access the data directly from the rrd file.