Spine is a tool for creating 2D skeletal animations for Unity (and many other game engines).
Unfortunately, Spine is NOT compatible with Animancer.
This is because Spine doesn't export "real" Unity animations.
A real animation contains all its relevant animation data which can be seen in Unity's Animation window:
But Spine animations are only empty dummies containing no real data:
Spine works by continuously checking which dummy animation the Animator Controller is playing at runtime and then applying the appropriate animation data which is stored separately in Spine's own animation format.
Unfortunately, the implementation of Spine seems to be very tightly coupled with the Animator Controller system so while it should be theoretically possible to make a variant which integrates with Animancer instead, such an integration doesn't currently exist and would be non-trivial to develop and maintain.