Yes. A rest day can be locked just like any other day, and locking it keeps it a rest day.
This is useful when the plan has given you a rest day on a date you specifically want to keep free — locking it tells TrainAsONE not to schedule anything there when your plan next updates.
Locking a rest day is not the same as telling us you cannot run. See What is the difference between locking a day and skipping a run? for the distinction.