Could not import _ext module

[yangaonidaye123]

I just follow the insructions for pointnet++ installment，but there is a problem：Traceback (most recent call last):
File "scripts/train.py", line 21, in
from models.refnet import RefNet
File "/root/temp/ScanRefer/models/refnet.py", line 8, in
from models.backbone_module import Pointnet2Backbone
File "/root/temp/ScanRefer/models/backbone_module.py", line 9, in
from lib.pointnet2.pointnet2_modules import PointnetSAModuleVotes, PointnetFPModule
File "/root/temp/ScanRefer/lib/pointnet2/pointnet2_modules.py", line 21, in
import pointnet2_utils
File "/root/temp/ScanRefer/lib/pointnet2/pointnet2_utils.py", line 29, in
raise ImportError(
ImportError: Could not import _ext module.
Please see the setup instructions in the README: https://github.com/erikwijmans/Pointnet2_PyTorch/blob/master/README.rst