need a help with strange error" left bound of the range must be a constant "

when S4 => 
 if(r_by_2 = 1) then
 q <= q (m_size-r_by_2-1 downto 0) & q(m_size-1);
 else
 q(m_size-1 downto r_by_2) <= q (m_size - r_by_2 - 1 downto 0);
 q(r_by_2-1 downto 0) <= q(m_size-1 downto m_size - r_by_2);
 end if;

can be replaced by:

when S4 =>
  for i in 0 to m_size - 1 loop
    q(i0) <= q((i + r_by_2) mod m_size) ;
  end loop ;

This code synthesises (I checked!)

You may want to use a barrel shift/rotate component in stead (especially if m_size is not a power of 2 as the code above then needs dividers to calculate the 'mod' function), it will probably use less resources and certainly run faster.