Failed to pull the image.
Sorry the FAQ refers to an image that doesn’t exist, I also had the pull issue and fixed it but then I pasted you the old command 
https://hub.docker.com/r/rancher/k3s/tags?page=1&ordering=last_updated&name=v1.20.6-k3s
k3d cluster create \
--k3s-server-arg "--kube-proxy-arg=conntrack-max-per-core=0" \
--k3s-agent-arg "--kube-proxy-arg=conntrack-max-per-core=0" \
--image rancher/k3s:v1.20.6-k3s1 # added a 1 to the end here lol
You can just do this too and let it pick an image:
k3d cluster create \
--k3s-server-arg "--kube-proxy-arg=conntrack-max-per-core=0" \
--k3s-agent-arg "--kube-proxy-arg=conntrack-max-per-core=0"
(also when doing k3d cluster delete you wont need these args as they dont exist, its just needed for creation)