How to add iTunes song to my iPhone?
I had the same problem, trying to use drag-and-drop (as I'd always done) to add a media file (audiobook) to the iPhone. 

When dragged to the iPhone, the item appeared in a small gray box with a red/white number such as "58." No idea what the number meant. In any case, I now know that the small gray box with the number is a visual cue that the dragged item will not be transferred to the phone.

After setting preferences to manual synch, this worked for me on a PC:


Connect the device to iTunes.
Select the device at the top of the screen (not in the sidebar). The iTunes Library view is replaced with your iphone view for "Settings" and "On My Device."
In Settings>Summary, scroll to Options and check-in (or out) Sync your synching method.
In Settings>Audibooks, check-in "Selected audiobooks." No need to actually select books from the list.
Switch back to regular iTunes Library (at the top of the screen, click the " &lt; " (back arrow).
From the Library, select the media type and show it in grid view.
Select the grid item (tile?) for the media file you want to transfer. 
Drag the item to the iphone. If drag-and-drop is working, then the entire iPhone and expanded content will be enclosed in a blue box.


Notes


After completing all of the steps, on the iphone, I had to leave "Synch" (see step 4, above) checked in. If I unchecked it, then all selected types of media were deleted from the iphone. 
My environment: iPhone 10; Windows 10; software for both is up-to-date.


Apple updates its applications' behaviors more frequently than I transfer files. Consequently, figuring out what's changed and writing the steps for yourself seems to never be sufficient.

Having said that, I hope this helps at least a few people in the short term.