multiplexed display system

Your problem is here:

else if rising_edge (slow_clk) then count_2b <= count_2b +1;

it should be:

elsif rising_edge (slow_clk) then count_2b <= count_2b +1;

This should fix the first of your compile issues. (you have more, but I will leave you to them)

Now that we have that fixed, I noticed you are using a gated clock. :evil: Gated clocks should be avoided if possible. Instead use clock enables. They look something like this:

process (rst,clk)
begin
    IF (rst = '1') THEN
        count =(OTHERS => '0');
        slow_clock_enable_s <= '0';
    ELSIF rising_edge(clk) THEN
        IF (count(15) = '1') THEN
            count =(OTHERS => '0');
            slow_clock_enable_s <= '1';
        ELSE
            count <= count +1;
            slow_clock_enable_s <= '0';
        END IF;
    end if;
end process;
process (rst, clk)
begin
    if rst = '1' then 
        count_2b <= "00";
    elsif rising_edge (clk) then 
        IF (slow_clock_enable_s = '1') THEN
            count_2b <= count_2b +1;
        END IF;
    end if;
end process;