Freeing up space · Emergency Space
Can Emergency Space help if my external drive isn't plugged in?
Short answer: Partly. Releasing held space and anything that needs no destination still work. Clear-ups keep a recoverable copy on your fast drive, so they're explained but not offered until you plug it in.
Partly, and it tells you where the limit is. Releasing space your Mac is holding still works, and so does anything that doesn't need somewhere to put things.
But clearing keeps a recoverable copy of whatever it removes, and that copy lives on your fast drive. With the drive unplugged the copy would have to sit on the same full disk, which would free nothing — so those options are explained rather than offered. Moving folders needs the drive too.
Plug the drive in and the rest of the plan becomes available. See where the Recycle Vault is kept.