This came up here, although not on emerald, but on codename 2e (i.e. the 2020 2e) …
If emerald distinguishes between a fastboot mode and a fastbootd mode like 2e seems to do, then make sure the phone is in fastboot mode when running the fastboot commands.
On other devices this error message came up if the fastboot command was too old, mostly fastboot commands available in Linux distributions were affected.
What does fastboot --version say?
Compare with the fastboot command from the current Android SDK Platform Tools .