39933

There are 2 solutions:
1.) 6, 9, 11, 12, 4, 14, 7.
2.) 8, 7, 13, 10, 6, 12, 9.

What you do is find all the solutions for fifteen not including a 0. Then you add the second number to another number to make 20 with numbers under 15 or lower not using any of the numbers in the previous. then repeat it to add the numbers up to 23. Then carry on for 16, 18 and 21.

Example:
start with
6+9=15
9+11=20
11+12=23
12+4=16
4+14=18
14+7=21
Sequence: 6, 9, 11, 12, 4, 14, 7.
The sequence is +2-2+2-2 repeated