Error 10818

You put the clock rising edge check inside an if check for an enable.

Your order for control signal checks for a register inference is incorrect. It should look something like this (you can also use a template in the Quartus text editor). Note that if you're trying to create a counter (not sure if that is your intention), anglecodeunsign is going to go back to the value of AngleCode every clock cycle:

PROCESS(clock, resetn)
BEGIN
	anglecodeunsign <= unsigned(AngleCode);
        IF resetn = '0' THEN
		anglecodeunsign <= "00000000";
	ELSIF rising_edge(clk) THEN
		IF enable = '1' THEN
			IF sclrn = '0' THEN
				anglecodeunsign <= "00000000";
			ELSE
				IF anglecodeunsign > "00010000" THEN
                                       anglecodeunsign <= "00000000"
                                ELSE
                                   anglecodeunsign <= anglecodeunsign + '1';
                                END IF;
			END IF;
		END IF;
	END IF;
END PROCESS;