Need help to figure out syntax error code

I've been following a few snippits given by professor to fill in gaps of code and now that I am finished I'm get and I keep getting error and can't figure out what its asking

Error (10500): VHDL syntax error at engine.vhd(30) near text "when"; expecting ";"

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;

entity engine is
port(
    RST, CLK, KEY0: in std_logic;
    Y3, Y2, Y1, Y0: out std_logic
);
end entity;

architecture behav of engine is
    type states is (RESET, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20);
    signal D: states;
    signal Q: states;
    
begin

    REG: process(all)
    begin
        if RST = '0' then Q <= RESET;
        elsif rising_edge(CLK) then Q <= D;
        end if;
    end process;
    
    NSL: process(all)
    begin
        D <= 
            T1  when Q = T0  else -- line 30
            T2  when Q = T1  else
            T3  when Q = T2  else
            T4  when Q = T3  else
            T5  when Q = T4  else
            T6  when Q = T5  else
            T7  when Q = T6  else
            T8  when Q = T7  else
            T9  when Q = T8  else
            T10 when Q = T9  else
            T11 when Q = T10 else
            T12 when Q = T11 else
            T13 when Q = T12 else
            T14 when Q = T13 else
            T15 when Q = T14 else
            T16 when Q = T15 else
            T17 when Q = T16 else
            T18 when Q = T17 else
            T19 when Q = T18 else
            T20 when Q = T19 else
            T0;
    end process;
    
    with Q select
        Y <=  B"1000" when T1  | T2  | T3  | T4  | T5,
                B"0100" when T6  | T7  | T8  | T9  | T10,
                B"0010" when T11 | T12 | T13 | T14 | T15,
                B"0001" when T16 | T17 | T18 | T19 | T20,
                B"0000" when others;
                
end architecture;