here is the error :
[ERROR] Failed to get sources of ocamlfind.1.9.6: curl error code 504
#=== ERROR while fetching sources for ocamlfind.1.9.6 =========================#
OpamSolution.Fetch_fail("http://download.camlcity.org/download/findlib-1.9.6.tar.gz (curl: code 504 while downloading http://download.camlcity.org/download/findlib-1.9.6.tar.gz)")