Galaxy S6 - Unable to Add Music

[Me-Hostage]

I have had music files (mp3) transferred from my computer to my phone via USB for a while. I went to clean off all the music tonight, and re-add what albums I wanted back, but now it's not letting me copy anything from my computer to my phone via USB? When I click copy and paste it just does the sound effect like you're clicking on something that can't open and it copies nothing?

 

[B. Diddy]

When you connect via USB, are you swiping down the notification panel, tapping the USB icon, and making sure MTP is selected? Can the computer see all of the files in the phone's user directories?