I believe this may be related to the fact that legacy dashboards built in Reports are slatted to be sunset at the end of the year. So pre-existing dashboards that were already scheduled can still be manipulated (to some degree), but they have likely shut down the ability to schedule new dashboards so as to discourage users from making reports into an architecture that will be gone by the end of the year.
At this point, it would make sense to start building new reports (and converting your existing reports) using Workspaces, so that you don't lose anything when these features will be shut down fully.
