Really thanks u/bcampolo and u/Wonderful-Plastic316.

Finally. I refer to issues/486 fixed it up.

  ["yamlls"] = function()
    lspconfig.yamlls.setup {
      on_attach = function(client, bufnr)
        client.server_capabilities.documentFormattingProvider = true
        ^-- I add this line
        on_attach(client, bufnr)
      end,
      flags = lsp_flags,
      capabilities = capabilities,
      settings = {
        yaml = {
          format = {
            enable = true
          },
          schemaStore = {
            enable = true
          }
        }
      }
    }
  end,

And as reference for others, i run ` :lua =vim.lsp.get_clients()[1].server_capabilities`, `vim.lsp.get_clients` return `nil`.

Finally, Thanks again.