The plugin is installed, I double-checked.
But I found something, the error is located in ThirdPerson_AnimBP, which is the animation blueprint generated by the “Third Person” template (which I changed the parent class for “StateMachineABP”),
However, if I create a new animation blueprint inheriting from StateMachineABP and put the node “Blend State Machines” in it, the error does not appear and my node stay there.
Anyway, thank for the quick feedback, this was a strange behaviour but at least it can be easily fixed