Multiple Distinct Clocks Error

Thank you for everyone's help. I didn't really understand most of it, but I'll check out those references. Here is the way I finally solved the problem:

if(clock'EVENT and clock='1') then
			
			
			we <= '0';
			if(key(3)='1' and key(2)='1' and key(1)='1' and key(0)='1') then
				EnableKeyPress := '1';
			end if; 
			if(key(3)='0' and EnableKeyPress='1') then 
				EnableKeyPress := '0';
				if(conv_integer(ram_addr) = 16) then
					ram_addr <= "0000";
				else
				ram_addr <= ram_addr + "0001";
				end if;
			elsif(key(2)='0' and EnableKeyPress='1') then
				EnableKeyPress := '0';
				if(conv_integer(ram_addr) = 0) then
					ram_addr <= "1111";
				else
				ram_addr <= ram_addr - "0001";
				end if;
			elsif(key(1)='0' and EnableKeyPress='1') then 
				EnableKeyPress := '0';
				we <= '1';
				if(data_out = "11111111") then
					data_in <= "00000000";
				else
				data_in <= data_in + 1;
				end if;
			elsif(key(0)='0' and EnableKeyPress='1') then
				EnableKeyPress := '0';
				we <= '1';
				if(data_out = "00000000") then
					data_in <= "11111111";
				else
				data_in <= data_in - 1;
				end if;
			end if;
		end if;