compiling error

You didn't post the declaration of the vector count_b, but I guess:

signal count_b : std_logic_vector(4 downto 0);

you can't add vector of type "std_logic_vector" ( width some library you can do it, but isn't a good practice ). The to do it is:

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity.....

architecture...

signal count_b : unsigned(4 downto 0);

process...

count_b <= count_b + 1;

If you need count_b as a std_logic_vector type you may cast it:

count_2 <= std_logic_vector(count_b);

( count_2 is an arbitrary name )