Append Existing Custom Node Group in Blender - python-3.x

Question

I tried to append my blend file and import the node group in my new environment. I tried different methods on how to append my own custom node group in my environment, and still not working. Here's my boilerplate.
class SHADER(Operator):
bl_idname = "material.append_shader_nodes"
bl_label = "Add Shader"
bl_options = {'REGISTER', 'UNDO'}
def execute(self,context):
# Importing the blend file (working)
import_from_library('shader');
bpy.ops.object.material_slot_add()
# Creates new Material
npr_material = bpy.data.materials.new(name='SHADER')
npr_material.use_nodes = True
# Remove the default shader
npr_material.node_tree.nodes.remove(npr_material.node_tree.nodes.get('Principled BSDF'))
material_output = npr_material.node_tree.nodes.get('Material Output')
# Problem
SHADER = bpy.data.node_groups['NPREEVEE'] # Import my custom node group from my different blend file
# link shader to material
npr_material.node_tree.links.new(material_output.inputs[0], SHADER.outputs[0])
# set activer material to your new material
bpy.context.object.active_material = npr_material
return {'FINISHED'}
It seems like it didn't import my node group, but when I tried to manually add my custom node group, it displayed on my materials properties. I'm not totally familiar with this package. Is this is a bug or there is something that I missed while creating my node group?