。
We're one relay baton short.
Literal
Relay-use [possessive-の] baton [subject-が] one-long-object insufficient [explanatory-んです].
~用 is a suffix meaning 'for use in.' 足りない means 'insufficient, not enough.' The counter ~本 is used for long, cylindrical objects — batons, pencils, bottles. なんです provides an explanatory tone, reporting a problem.