Looks like you are using french version of Excel, so I will leave it up to you to find the translated commands. The basic problem from what I am gathering is you are looking up "1/1" in the named range NumEchelon. I am going to guess that this named range is 1 column wide. sheet2[Indice] is a separate chunk of data.
Vloopkup is supposed to search for a specified term in a table (usually 2 or more columns wide located to the right of the search column) and return a value in the same row as the found value in a specified column. The columns are numbered left to right with the first column being zero. The false or FAUX at the end tells it you want an exact match.
So without seeing your data I would say make sure that NumEchelon covers both columns of information, and INDICE column is to the right of NumEchelon. Where you have Sheet2 Indice replace with with a numeric value for the column its from the table you made for the vlookup.
So Assuming NumEchelon is A1:A8 and Indice is B1:B8 I would do the following:
Use a new named range "MonTableaux" and define it as sheet2!A1:B8
=VLOOKUP("1/1",MonTableaux,2,0)
Without the named range it would look like
=VLOOKUP("1/1",sheet2!$A$1:$B$8,2,0)
'note the 0 is the same as false
Now you may be using TABLES and I am not all that familiar with table so there may be short cuts. If that is the case someone point it out to me and I will delete my answer.
If your information you want to return in not lined up vertically with the information you are searching for or if the information you want to return is located to the left of what you are searching for you will want to use a combination of INDEX and MATCH. maybe something like this:
=INDEX(sheet2[Indice],match("1/1",sheet2[NumEchelon],0))
#N/A
? – Hart CO