Hi Youjin,
I was able to resolve my problem by updating to the latest versions of emmet-core and mp_api with the commands
pip install emmet-core --update
pip install mp_api --update
and then access the elasticity data with
with MPRester(my_api_key) as mpr:
docs = mpr.materials.summary.search(fields=["bulk_modulus", "shear_modulus"])
bulk_modulus_voigt = doc.bulk_modulus["voigt"]
bulk_modulus_reuss = doc.bulk_modulus["reuss"]
bulk_modulus_vrh = doc.bulk_modulus["vrh"]
shear_modulus_voigt = doc.shear_modulus["voigt"]
shear_modulus_reuss = doc.shear_modulus["reuss"]
shear_modulus_vrh = doc.shear_modulus["vrh"]