Hi Dorothée,
I can't see what process options you chose from your report, but if you want to average all of the input files together you need to make sure you select "Group files: Everything" (the very first option).
The different options available in the Average process are detailed in the following tutorial: https://neuroimage.usc.edu/brainstorm/Tutorials/Averaging#Process_options:_Average
I hope this helps,
Martin