Are you trying to copy and paste it using a File Manager on your phone or on your computer?
If both and it still doesn't work then can you copy the file and paste it to the "Notifications" folder?
Are you able to connect your phone to your computer and move a file to the "Notifications" folder? If so then might be worth trying to move the file to your computer and back again.
Also, doesn't it have to go in the Media > audio > ringtones folder?
I don't have mine in there so can't remember where the official place is.
These are the first things I'd try, if none of this works then let us know and we'll try and figure it out