Hi Khoa,
The specific notebook you linked to is for DL part 2 version 2 of the course. The notebook is for lesson 9 multi-object detection.
fastai v1.0 library is currently designed to support part 1 version 3 syllabus and concepts. fastai 1.0 does not support object detection and multi-object detection yet. Ref: Object detection in fast.ai v1
The question is, why are you using notebooks from part 2 with fastai v1.0? You have better chances to get things work if you use notebooks from part 1.