You can write Fennel code in any editor, but some editors make it more comfortable than others. Most people find support for syntax highlighting, automatic indentation, and delimiter matching convenient, as working without these features can feel tedious.
Other editors support advanced features like an integrated REPL, live reloading while you edit the program, documentation lookups, and jumping to source definitions.
If your favorite editor isn't listed here, that's OK; stick with what you're most comfortable with. You can usually get decent results by telling your editor to treat Fennel files as if they were Clojure or Scheme files.
These are all 3rd-party projects; mentions here are not endorsements.