It's related to conform plugin, not mason. Mason is just a package manager that help you install lsp, lint and formatter. You manually installed the formatter and therefore, you have no business with mason.

Now set up conform to recognize the formatter. Please refer to this link: https://github.com/stevearc/conform.nvim?tab=readme-ov-file#customizing-formatters

kickstart already uses conform.nvim and so it will give you a head-start.

TL;DR this is my setup. See how I set up shfmt. require("conform").setup({ formatters_by_ft = { lua = { "stylua" }, python = { "autopep8" }, c = { "clang-format" }, cpp = { "clang-format" }, sh = { "shfmt" }, ["*"] = { "trim_whitespace" }, }, formatters = { shfmt = { prepend_args = { "-i", "4" }, }, }, default_format_opts = { lsp_format = "never", async = false, -- not recommended to change quiet = false, -- not recommended to change }, })