Hello, it seems to be a bug of your plugin. The problem is when for example. The taxonomy has some term nested in 3 levels, but it also has other nested terms in only two levels. Then The validation of the abligatory field fails because apparently it is waiting for a third level, even though it should not be like that.
Please look at my video, you will see that the term “Sab” has no children, so there is no way to select a third level.