Postgresql - Error in function that returns table
You don't need plpgsql for this. A plain sql function will do: 

create or replace function GET_COLUMNS(in tbl_name character varying(30))
   returns table(colunas character varying) 
as 
$$
    SELECT column_name
    FROM information_schema.columns
    WHERE table_schema = 'Main'
    AND table_name   = tbl_name;
$$ 
language sql;


Additionally: the language name is an identifier. Don't put it in single quotes.