Daniele,


MSG seviri instrument has these channels:

# Channel | VIS006 |   HRV  | VIS008 | IR_016 | IR_039 | WV_062 |
# WaveLen |  0.635 |   0.7  |   0.81 |   1.64 |   3.90 |   6.25 |
# --------+--------+--------+--------+--------+--------+--------+
# Channel | WV_073 | IR_087 | IR_097 | IR_108 | IR_120 | IR_134 |
# WaveLen |   7.35 |   8.70 |   9.66 |  10.80 |  12.00 |  13.40 |


Satpy composite definition of 'airmass':
...
  airmass:
    compositor: !!python/name:satpy.composites.GenericCompositor
    prerequisites:
    - compositor: !!python/name:satpy.composites.DifferenceCompositor
      prerequisites:
      - wavelength: 6.2
      - wavelength: 7.3
    - compositor: !!python/name:satpy.composites.DifferenceCompositor
      prerequisites:
        - wavelength: 9.7
        - wavelength: 10.8
    - wavelength: 6.2
    standard_name: airmass
...


You need two more 'airmass' lines in your script, put it like below:

...
# We only decompress what is actually needed, see file seviri.yaml
needed_files = {'overview'         : ['VIS006', 'VIS008', 'IR_108'],
                'natural_color'    : ['VIS006', 'VIS008', 'IR_016'],
                'realistic_colors' : ['VIS006', 'VIS008', 'HRV'],
                'hrv_clouds'       : ['IR_108', 'HRV'],
                'night_fog'        : ['IR_039', 'IR_108', 'IR_120', 'IR_134'],
                'airmass'          : ['WV_062', 'WV_073', 'IR_097', 'IR_108'],
                'ir108_3d'         : ['IR_108']}
# Some composite names are too long for IM annotation left of pic
abbreviation = {'overview'         : 'overview',
                'natural_color'    : 'natural_color',
                'realistic_colors' : 'realist_color',
                'hrv_clouds'       : 'hrv_clouds',
                'night_fog'        : 'night_fog',
                'airmass'          : 'airmass',
                'ir108_3d'         : 'ir108_3d'}
...

Then set composite='airmass' and run the script ...


Hope this helps,

Ernst