ERROR: query has no destination for result data: Postgresql - postgresql

Question

I am using Postgresql11 and a function that works well in a single run fails when I add a LOOP statement with
"ERROR: query has no destination for result data HINT: If you want to
discard the results of a SELECT, use PERFORM instead."
The function has VOID as return value, selects data from a source table into a temp table, calculates some data and inserts the result into a target table. The temp table is then dropped and the function ends. I would like to repeat this procedure in defined intervals and have included a LOOP statement. With LOOP it does not insert into the target table and does not actually loop at all.
create function transfer_cs_regular_loop(trading_pair character varying) returns void
language plpgsql
as
$$
DECLARE
first_open decimal;
first_price decimal;
last_close decimal;
last_price decimal;
highest_price decimal;
lowest_price decimal;
trade_volume decimal;
n_trades int;
start_time bigint;
last_entry bigint;
counter int := 0;
time_frame int := 10;
BEGIN
WHILE counter < 100 LOOP
SELECT max(token_trades.trade_time) INTO last_entry FROM token_trades WHERE token_trades.trade_symbol = trading_pair;
RAISE NOTICE 'Latest Entry: %', last_entry;
start_time = last_entry - (60 * 1000);
RAISE NOTICE 'Start Time: %', start_time;
CREATE TEMP TABLE temp_table AS
SELECT * FROM token_trades where trade_symbol = trading_pair and trade_time > start_time;
SELECT temp_table.trade_time,temp_table.trade_price INTO first_open, first_price FROM temp_table ORDER BY temp_table.trade_time ASC FETCH FIRST 1 ROW ONLY;
SELECT temp_table.trade_time,temp_table.trade_price INTO last_close, last_price FROM temp_table ORDER BY temp_table.trade_time DESC FETCH FIRST 1 ROW ONLY;
SELECT max(temp_table.trade_price) INTO highest_price FROM temp_table;
SELECT min(temp_table.trade_price) INTO lowest_price FROM temp_table;
SELECT INTO trade_volume sum(temp_table.trade_quantity) FROM temp_table;
SELECT INTO n_trades count(*) FROM temp_table;
INSERT INTO candlestick_data_5min_test(open, high, low, close, open_time, close_time, volume, number_trades, trading_pair) VALUES (first_price, highest_price, lowest_price, last_price, first_open, last_close, trade_volume, n_trades, trading_pair);
DROP TABLE temp_table;
counter := counter + 1;
SELECT pg_sleep(time_frame);
RAISE NOTICE '**************************Counter: %', counter;
END LOOP;
END;
$$;

Answer 1

The error refers to the last SELECT statement in the function. If there is a SELECT without INTO it will always return results. When there's no LOOP this result will be used as the return value of the function (even if it is void).
When you add a LOOP there can't be any SELECT without INTO inside the loop because a single return value would be needed and now there will be many. In this case you need to use PERFORM which does exactly the same thing as a SELECT but discards the results.
Therefore change the last SELECT into a PERFORM and the error will go away:
PERFORM pg_sleep(time_frame);