Inability to exchange units between armies (Total War: Warhammer 2)

Good day, I ask for help in difficulty. The next moment
arose. I'm playing for the skaven.
A legendary hero with an army of 19/20, located in the location of the foul squabbles.
Second sorcerer master 8/20 ibid.
I want to give him some of the troops, and I myself hire the legendary
BUT the hero runs up and runs away nothing happens.

Please tell me what I'm missing