Consider the following statement:

create function get_most_frequent_topic(out topic_id integer)
as $$
begin
    select 
        p.topic_id into topic_id,
        count(*) as freq
    from post p
    group by p.topic_id
    order by count(*) desc
    limit 1;
end;
$$ language plpgsql

When I execute, I get this error:

Error occurred during SQL query execution

Reason:
SQL Error [42601]: ERROR: "count" is not a known variable
  Position: 123

However,if I remove count(*) from select, then it starts working. Why?

create function get_most_frequent_topic(out topic_id integer)
as $$
begin
    select 
        p.topic_id into topic_id        
    from post p
    group by p.topic_id
    order by count(*) desc
    limit 1;
end;
$$ language plpgsql