Impossible upgrade from Sierra to High Sierra

Hello,

i'm French sorry for my english.

I am owner of imac 2010 who is compatible with High Sierra.

My actual version of macOS is 10.12.6 (Sierra) and i'm trying tu update to 10.13 (High Sierra)

I tried to do it with this link :

https://support.apple.com/fr-fr/HT208969

but when i want to download mac os high Sierra (step 4), Itunes is opening and i can read

"The item you are looking for is currently not available in the Store“

I can't either download it on the Appstore.

What can i do?