$ opam update

<><> Updating package repositories ><><><><><><><><><><><><><><><><><><><><>  🐫
[default] no changes from https://opam.ocaml.org
[ERROR] Could not update repository "default": In
        /private/var/folders/x8/8s64cxbx0yj16d3shdxmm5gr0000gn/T/opam-12814-5e0534/default/packages/ocaml-variants/ocaml-variants.5.0.0+trunk/opam:
        unsupported or missing file format version; should be 2.0 or older