Can you please help me out with the following problem:

i try to impliment this cod and ther is that Error (10822): HDL error at XOU.vhd(13): couldn't implement registers for assignments on this clock edge

library ieee;
use ieee.std_logic_1164.all;
 
 
entity XOU is
port (a,h,r,b:in std_logic;s:out std_logic);
end XOU;
architecture AXOU of XOU is
begin
process(h,r)
begin
if r='0' then s<='0';
elsif falling_edge(h)then
s<='1';
else s<='0';
end if ;end process;end AXOU;