Chapter 6: Aliens moving head

I have the following problem which I’ve tried twice now.
In Chapter 6 p.162-163 we make the alien head move as an animation. When I follow the instructions and then play the game the alien heads move up and down as wanted, however, I get the following errors and the aliens do not move anymore, like they are stuck to the ground, even though they did move before:

“Failed to create agent because it is not close enough to the NavMesh”
“SetDestination” can only be called on an active agent that has been placed on a NavMesh

Any ideas why this is happening?

Thank you