Q: Unable to manually transfer music from iTunes to iPhone 6s

Hello bcarol ,

Thank you for being part of the Apple Community!

For troubleshooting the issue of manually moving music to your iPhone from iTunes, I have listed below an article that outlines all the steps.  Is it possible that you are trying to copy from a different iTunes library then what was used before?  As mentioned in the article, it has to be the same iTunes library that has been used in the past.

Move content from iTunes on your computer to your iPhone, iPad, or iPod touch

Cheers!

Hi castle11,

The next thing I would recommend would be to be sure that iTunes is running the latest version.  To check that iTunes is up to date follow these steps:

1. Open iTunes.
2. From the menu bar at the top of the iTunes window, choose Help > Check for Updates. Learn what to do if you can’t see the menu bar.
3. Follow the prompts to install the latest version.

Once the latest version is installed open iTunes, and try to transfer the music again.

If the issue is still occurring, then removing iTunes and re-installing should help.  Before doing this, be sure to back up the computer to ensure that no data is lost.

Follow the steps outlined here to remove iTunes:  Remove and reinstall iTunes and related software components for Windows 7 and later

Have a great day!