Error: No module named 'datasets'

[Ishihara-Masabumi]

When I run the followint command line, the following error occurred.

$ python3 datasets/preprocess.py
Traceback (most recent call last):
  File "datasets/preprocess.py", line 4, in <module>
    from datasets.bvh_parser import BVH_file
ModuleNotFoundError: No module named 'datasets'

Please let me know how to fix it.