You issued the command on context. The target context has some task(s) that are RUNNING. This is an informational message to let you know that a pdbx prompt will not be displayed immediately.
If you want to switch to a context where all tasks are ready to be debugged, press <Ctrl-c> to obtain a pdbx-subset prompt. You can list the state of tasks using the tasks or group list command. The on command is available at the pdbx-subset prompt if you want to then switch to a different context.