This channel is yet to be defined although it is mentioned in our documentation and 4.15.0 is GA .    

https://docs.openshift.com/container-platform/4.15/updating/updating_a_cluster/updating-cluster-cli.html#:~:text=oc%20adm%20upgrade%20channel%20stable%2D4.15

  100%  

 1.deploy 4.14.0 ga version, I was using this job: https://auto-jenkins-csb-kniqe.apps.ocp-c1.prod.psi.redhat.com/job/CI/job/job-runner/2414/

   2. flow the above documented procedure :
[kni@ocp-edge119 ~]$ oc adm upgrade channel stable-4.15

  [kni@ocp-edge119 ~]$ oc adm upgrade channel stable-4.15
error: the requested channel "stable-4.15" is not one of the available channels (candidate-4.14, candidate-4.15, eus-4.14, fast-4.14, fast-4.15, stable-4.14), you must pass --allow-explicit-channel to continue
  

[kni@ocp-edge119 ~]$ oc adm upgrade channel stable-4.14
[kni@ocp-edge119 ~]$ oc adm upgrade channel stable-4.14
info: Cluster is already in stable-4.14 (no change)