Here are steps required to be able to transfer files from the PC to the Android device.

 After you plug the phone into the computer (be sure to use the USB cable that came with your Android device), on some phones, you have to enable file transfer manually in order to drag and drop files.  On some devices, the default is just to allow the computer to charge the phone and blocks file transfers.  Here are some screenshots:

After plugging in the USB cable to the computer, navigate to the pull down menu/notification bar, tap on the “Android System   USB for…” option, tap it again, and ensure that “Transferring files / Android Auto” is selected.