I have downloaded an app called Mezurio which won't function without notifications.  However the app does not appear under Notifications in Settings.  When I choose "all system apps" it does then show, but won't allow me to switch on notifications - it stays greyed out and gives the message in the subject title above.

I have the latest Android software and the app is the current version.  I have rebooted the phone and uninstalled and reinstalled the app.

Has anyone any bright ideas, please?