MaxString({<Année={2016},[Numéro de prospectus]={$(=subfield(concat( {<Année={2016}>}DISTINCT [Numéro de prospectus],';'),';',2))}>}[Libellé de prospectus])
What is the problem? You see no data? Is there any error in the object? Is it a text object or a table?
Two things I can think of without more background:
If you are expecting a string (not a number) then you need to use a single quote for the Numéro de prospectus field:
MaxString({<Année={2016},[Numéro de prospectus]={"$(=subfield(concat( {<Année={2016}>}DISTINCT [Numéro de prospectus],';'),';',2))"}>}[Libellé de prospectus])
Also, try the SubField() function separately and make sure it returns as expected, one single value
EDIT: Set to double quotes because of the use of single quotes already in the functions.