Augmentations for the global scope can only be directly nested in external modules or ambient module declarations(2669)

You can indicate that the file is a module like so:

export {};

declare global {
    namespace NodeJS {
        interface Global {
            config: MyConfigType
        }
    }
}

Or if you're trying to add a global type within the browser context:

export {};

declare global {
  interface Window {
    ENV: any;
  }
}