Samsung S-7 - Windows 10 Data Transfer (music)

Again I am reaching out the the experts on RV.NET for assistance. I am trying to transfer some music files (MP3's) from my Windows 10 PC into my Samsung S-7 phone using the USB cable. I know I can send each song as an email attachment to the phone email program and then download the attachment but this is time consuming. I also know that it can be done using Bluetooth between the WIN 10 PC and S-7 for multiple files at once. I am trying to use the USB cable method. I connect the phone to the PC using the USB cable and the PC recognizes it.. but when I try to send the file to the phone I get the message " CANNOT COPY ITEM.. The device has either stopped responding or has been disconnected"... Then I tried the Kies 3 program to do the same and I get the message that the S-7 is locked and I have to enter an unlock code on the phone..(No idea what this means).. Any ideas? Maybe my Bluetooth method is the only way for me to do this but it should also be possible using the USB cable. My neighbor who has a WIN 7 PC and a Galaxy S-6 has experience the same problem BUT his computer doesn't have Bluetooth.. Not a big deal but curiosity on how to do it is getting me.. Thanks