Hi llealloo,
I installed usd-core at C4D R26 by powershell in this folder \Maxon Cinema 4D R26\resource\modules\python\libs\python39.win64.framework, and the following test code works fine and can produce the output usd file.
Hope it may give you more clue.

import c4d
import pxr
# following code from GTC_usd_introduction.ipynb------------------------------
from pxr import Usd, UsdGeom
# Create a tempory stage in memory
stage = Usd.Stage.CreateInMemory('SampleLayer.usda')

# Create a transform and add a sphere as mesh data
xformPrim = UsdGeom.Xform.Define(stage, '/MySphere')

# Set a translation
UsdGeom.XformCommonAPI(xformPrim).SetTranslate((7,8,9))

spherePrim = UsdGeom.Sphere.Define(stage, '/MySphere/MeshData')

# Get the sphere as a generic prim
sphere = stage.GetPrimAtPath('/MySphere/MeshData')

# Get the extent and radius parameters for the prim
radiusAttr = sphere.GetAttribute('radius')
extentAttr = sphere.GetAttribute('extent')

# Access the sphere schema to set the color
colorAttr = spherePrim.GetDisplayColorAttr()

# Set the radius to 2
radiusAttr.Set(2)

# Expand the extents to match the new radius 
extentAttr.Set(extentAttr.Get()*2)

# Make the sphere blue
colorAttr.Set([(0,0,1)])

# Print out the stage
print(stage.GetRootLayer().ExportToString())

# Save the resulting layer
stage.GetRootLayer().Export('SampleLayer.usda')