Ocaml: opam install ppx_deriving_yojson not working

I want to install the library ppx_deriving_yojson for OCaml with OpamI have tried:

opam install ppx_deriving_yojson

The error message

[NOTE] It seems you have not updated your repositories for a while. Consider updating them with:       opam updateThe following actions will be performed:∗ install ppx_derivers        1.2.1   [required by ppx_deriving]∗ install sexplib0            v0.15.1 [required by ppxlib]∗ install ocaml-compiler-libs v0.12.4 [required by ppxlib]∗ install ppxlib              0.28.0  [required by ppx_deriving_yojson]∗ install ppx_deriving        5.2.1   [required by ppx_deriving_yojson]∗ install ppx_deriving_yojson 3.7.0===== ∗ 6 =====Do you want to continue? [Y/n] Y<><> Gathering sources ><><><><><><><><><><><><><><><><><><><><><><><><><><><><>[ERROR] The sources of the following couldn't be obtained, aborting:          - ocaml-compiler-libs.v0.12.4: curl error code          - ppx_derivers.1.2.1: curl error code          - ppx_deriving.5.2.1: curl error code          - ppx_deriving_yojson.3.7.0: curl error code          - ppxlib.0.28.0: curl error code          - sexplib0.v0.15.1: curl error code

Can anyone help me?My Opam version is

$ opam --version2.0.10