Can't add custom ringtones to iPhone 7. Help?

Hi

I know there have been other posts with the same issue, but....those don't seem to solve my problem.

I've added custom ringtones to my iPhones in the past, it can be a bit fussy but it eventually works.

I know there are Apple postings that say to drag and drop (or copy and paste) the ringtones into the sidebar on the iPhone's Tones folder (in iTunes). But that simply doesn't work. Won't copy. Nothing happens

If I try to copy or paste into the main Tones window, I get the error message "XXX was not copied to the iPhone because it cannot be played on this iPhone".

How can I copy the ringtones into the Tones folder for the iPhone when the normal way just doesn't work? Or what am I doing wrong?

Thanks.