what am I doing wrong here ?

create or replace function latestOilFee()
return array
IS
Begin
select * from oil_tracker order by ts desc fetch first 1 row only;
End;

Error: syntax error at or near "return"

An array is a container for other data types, and postgres doesn't know what you expect the result to be. This should work:

sql
CREATE OR REPLACE FUNCTION latestOilFee()
RETURNS SETOF oil_tracker
AS $BODY$
BEGIN
    RETURN QUERY
    SELECT *
    FROM oil_tracker
    ORDER BY ts DESC
    LIMIT 1;
END;
$BODY$ LANGUAGE plpgsql

interesting I thought array is also a data type