I know what this is, I remember finding this a few months ago when I was bored.

For some really weird reason, the bottom half of the ninja selection is blocked, or at least a portion of it.

As long as you have 1 ninja already selected, you will not be able to pick a good portion of the bottom ninjas. It doesn't matter who you pick, you could pick Enma or Cat god 1/2, it'd still say they were already selected. I don't quite know how to fix this though, it'd probably be best to report this problem to bugs/services.