If installing using

kubectl create -f kubeless.yaml --namespace kubeless

gives the following error:

customresourcedefinition "" created error: error validating
"kubeless.yaml": error validating data: unknown object type 
schema.GroupVersionKind{Group:"", Version:"v1", Kind:"Service"}; if you
choose to ignore these errors, turn validation off with --validate=false

You probably have an older version of Kubernetes. Make sure you are using at least version 1.7.