CharacterMesh has to have 'Simulate Physics' enabled if you'd like to AddImpulseAtLocation

The ‘Is Simulating Physics’ node fixed this error for me.
Seems to work for both skeletal and static meshes.

Edit:
Realized I needed to include the ‘Hit Bone’ from the hit result - without it the ‘Is Simulating Physics’ node was never true for Skeletal Meshes, and ‘Add Impulse’ needs it to work too.