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

gcloud notebooks instances create example-instance \
    --vm-image-project=deeplearning-platform-release \
    --vm-image-family=pytorch-1-7-cu110-notebooks \
    --machine-type=n1-standard-4 \
    --location=us-central1-a \
    --boot-disk-size=100 \
    --accelerator-core-count=1 \
    --accelerator-type=NVIDIA_TESLA_T4 \
    --install-gpu-driver \
    --network=default

At that time, I was unable to create the notebook instance due to the following error.

ERROR: (gcloud.notebooks.instances.create) The zone 'projects/{project}/zones/us-central1-a' does not have enough resources available to fulfill the request.  '(resource type:compute)'.

Therefore, I changed the location from us-central1-a to us-central1-b, and was able to create the instance.

gcloud notebooks instances create example-instance \
    --vm-image-project=deeplearning-platform-release \
    --vm-image-family=pytorch-1-7-cu110-notebooks \
    --machine-type=n1-standard-4 \
    --location=us-central1-b \
    --boot-disk-size=100 \
    --accelerator-core-count=1 \
    --accelerator-type=NVIDIA_TESLA_T4 \
    --install-gpu-driver \
    --network=default

I hope this helps others facing the same issue.