The following works as (I) would expect and ends up with a table consisting of a single time(7)
column:
create table T (Departure_time nvarchar(10) not null)
insert into T(Departure_time) values
('06:44:00'),('06:45:00'),('06:45:00'),('06:46:00'),('06:47:00'),
('06:47:00'),('06:48:00'),('06:49:00'),('06:50:00'),('06:50:00'),
('06:50:00'),('06:51:00'),('06:51:00'),('06:52:00'),('06:52:00')
alter table T alter column Departure_Time time(7) not null
I suspect you have some rows which don't have valid values in them. These may be more or less easy to discover, depending on how close to the required format the invalid values are.
select * from t where not Departure_time like '[0-2][0-9]:[0-5][0-9]:[0-5][0-9]'
Finds values that are definitely wrong. But it wouldn't find e.g. 27:34:32