Can not run digits

im following this tutorial in order to download Digits on Ubuntu 18.04

but in the step :
nvidia-docker run --name digits -d -p 8888:5000 -v /home/username/data:/data:ro -v /home/username/digits-jobs:/workspace/jobs nvcr.io/nvidia/digits:18.05

it gives me this error:
Unable to find image ‘nvcr.io/nvidia/digits:18.05’ locally
18.05: Pulling from nvidia/digits