Fill Missing Dates for Each Group in an existing table

Vertica has a clause that has been invented for something like what you ask the TIMESERIES clause.
In your case, you would be tempted to use it as:
TIMESERIES gen_date AS '1 DAY' OVER(PARTITION BY branch,business_unit ORDER BY "date")
But then, it would start and end the series for 'CE01' and 15001 with '2020-12-01' and '2020-12-10', and the other with '2020-12-02' and '2020-12-08'.

So you have practially no other choice than to add a distinct timeseries in an in-line table, and CROSS JOIN that with a SELECT DISTINCT of branch and business_unit from your base table.

What you might lose with this approach is any automatic constant or linear interpolation of any measures (which you're not including in your example though) ...

Here goes:

WITH
-- your input ...
indata(dt,brnch,bus_unit) AS (
          SELECT DATE '2020-12-01','CE01',15001
UNION ALL SELECT DATE '2020-12-05','CE01',15001
UNION ALL SELECT DATE '2020-12-02','CE03',15002
UNION ALL SELECT DATE '2020-12-08','CE03',15002
)
-- real query starts here; replace "," with "WITH"
,
l(l_dt) AS (
            SELECT MIN(dt)::TIMESTAMP FROM indata
  UNION ALL SELECT MAX(dt)::TIMESTAMP FROM indata
)
,
ts AS (
  SELECT
    dt::DATE AS dt
  FROM l
  TIMESERIES dt AS '1 DAY' OVER(ORDER BY l_dt)
)
,
dist AS (
  SELECT DISTINCT
    brnch
  , bus_unit
  FROM indata
)
SELECT
  dt
, brnch
, bus_unit
FROM dist
CROSS JOIN ts
ORDER BY 2,1
;
-- out      dt     | brnch | bus_unit 
-- out ------------+-------+----------
-- out  2020-12-01 | CE01  |    15001
-- out  2020-12-02 | CE01  |    15001
-- out  2020-12-03 | CE01  |    15001
-- out  2020-12-04 | CE01  |    15001
-- out  2020-12-05 | CE01  |    15001
-- out  2020-12-06 | CE01  |    15001
-- out  2020-12-07 | CE01  |    15001
-- out  2020-12-08 | CE01  |    15001
-- out  2020-12-01 | CE03  |    15002
-- out  2020-12-02 | CE03  |    15002
-- out  2020-12-03 | CE03  |    15002
-- out  2020-12-04 | CE03  |    15002
-- out  2020-12-05 | CE03  |    15002
-- out  2020-12-06 | CE03  |    15002
-- out  2020-12-07 | CE03  |    15002
-- out  2020-12-08 | CE03  |    15002

Thank You @marcothesane . I used the Cross Join logic and it worked. Thanks a lot!!!