Issue of structure of query does not match function result type
To return a TABLE add the table definition:

CREATE OR REPLACE FUNCTION historicalavg01() 
RETURNS TABLE
(
    Hist double precision,
    Tm   timestamp
) AS
$$
DECLARE starttime timestamp;
DECLARE endtime timestamp;
DECLARE total_hour_avg DOUBLE PRECISION;
l_row RECORD;
BEGIN
CREATE TEMP TABLE historical_temp(Histroical DOUBLE PRECISION,time timestamp);
total_hour_avg := 0;
FOR b IN 0 .. 23 LOOP
    FOR a IN 0 .. 27 LOOP

        starttime := (CURRENT_DATE - (a  * INTERVAL '1 day')) + (b * INTERVAL '1 hour');

        endtime := (CURRENT_DATE - (a  * INTERVAL '1 day')) + ((b + 1) * INTERVAL '1 hour');

        --total_hour_avg := total_hour_avg + (SELECT Avg("activepower") AS "Avg" FROM sca_meter WHERE "time" between starttime AND endtime);
        total_hour_avg := b * 0.5;

        IF(a = 27) THEN 
            INSERT INTO historical_temp SELECT AVG(total_hour_avg) AS "Histroical Avg", CURRENT_DATE - ((b + 1) * interval '23 hour') AS "time";
        END IF;

    END LOOP;
END LOOP;

RETURN QUERY SELECT * FROM historical_temp;

END$$ LANGUAGE plpgsql;
SELECT * FROM historicalavg01();




hist | tm                 
:--- | :------------------
0    | 2020-02-05 01:00:00
0.5  | 2020-02-04 02:00:00
1    | 2020-02-03 03:00:00
1.5  | 2020-02-02 04:00:00
2    | 2020-02-01 05:00:00
2.5  | 2020-01-31 06:00:00
3    | 2020-01-30 07:00:00
3.5  | 2020-01-29 08:00:00
4    | 2020-01-28 09:00:00
4.5  | 2020-01-27 10:00:00
5    | 2020-01-26 11:00:00
5.5  | 2020-01-25 12:00:00
6    | 2020-01-24 13:00:00
6.5  | 2020-01-23 14:00:00
7    | 2020-01-22 15:00:00
7.5  | 2020-01-21 16:00:00
8    | 2020-01-20 17:00:00
8.5  | 2020-01-19 18:00:00
9    | 2020-01-18 19:00:00
9.5  | 2020-01-17 20:00:00
10   | 2020-01-16 21:00:00
10.5 | 2020-01-15 22:00:00
11   | 2020-01-14 23:00:00
11.5 | 2020-01-14 00:00:00



db&lt;>fiddle here

Have a look at this tutorial about it.