I created a notebook instance using the following as a reference.
https://cloud.google.com/blog/ja/topics/developers-practitioners/pytorch-google-cloud-how-train-pytorch-models-ai-platform
At that time, I was unable to create the notebook instance due to the following error.
Therefore, I changed the location from us-central1-a to us-central1-b, and was able to create the instance.
I hope this helps others facing the same issue.