What that a first install?
Is it showing in your list of installed Rock-ons?
Alternatively, a docker ps -a may give some more information about what is happening here. Otherwise, you may have to “manually” delete the Rock-on from the cli using the delete_rockon call.
On another note, the Emby Rockon is due for an update. I’ve been putting it on the side as the current Json is still working but as Emby switched its inner workings quite a bit recently, it may help a lot. I’ll create an issue to remind myself.