problem with testbench

library IEEE;
use IEEE.std_logic_1164.all;
use STD.TEXTIO.all;
use IEEE.std_logic_unsigned.all;
use IEEE.std_logic_arith.all;


entity tb_file_sum_square_dif is
  generic
		(
			N :integer := 8
		);
end tb_file_sum_square_dif;


architecture TB_ARCHITECTURE of tb_file_sum_square_dif is
file IN_VECTORS: TEXT open READ_MODE is "input.txt";
file OUT_VECTORS: TEXT open WRITE_MODE is "output.txt";
--here
component sum_new_squaredif_n
    port(
     a,b: in signed(N-1 downto 0);
	   sum:out std_logic_vector(3*N-1 downto 0);
	   borout:out std_logic); --dn kserw posa einai to worst case
end component;

signal a,b: signed(N-1 downto 0);
signal sum:std_logic_vector(3*N-1 downto 0);
signal borout: std_logic;
begin
    UUT: sum_new_squaredif_n
        port map (
          a=>a,
          b=>b,
          sum=>sum,
          borout=>borout);
            
    process
        variable IN_BUF: LINE;
        variable OUT_BUF: LINE;
        variable a_var,b_var : bit_vector(N-1 downto 0);
        variable sum_var:bit_vector(3*N-1 downto 0);
    begin
        
        while not ENDFILE(IN_VECTORS) loop
            READLINE(IN_VECTORS,IN_BUF);
            READ(IN_BUF,a_var);
            READ(IN_BUF,b_var);
        read_inp: for k in 0 to N-1 loop
              a(k)<=to_stdulogic(a_var(k));
              b(k)<=to_stdulogic(b_var(k));
              end loop;
              wait for 1 ms;
            WRITE(OUT_BUF,STRING'("SUM OF SQUARE DIFFERENCE IS= "));
           -- sum_var:=to_bitvector(sum);
            WRITE(OUT_BUF,to_bitvector(sum));
            WRITELINE(OUT_VECTORS,OUT_BUF);
        end loop;
        --wait;
    end process;
    
end TB_ARCHITECTURE;

 Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 0  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 0 ns  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 1 ms  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 1 ms  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 2 ms  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT
# ** Warning: There is an 'U'|'X'|'W'|'Z'|'-' in an arithmetic operand, the result will be 'X'(es).
#    Time: 2 ms  Iteration: 1  Instance: /tb_file_sum_square_dif/UUT