If you can't drag and drop the music from Windows 10 computer to Galaxy S9 plus via MTP mode, my suggestion is pro Samsung mobile manager software. Such software like Coolmuster Android Assistant can help us to transfer music files from a Windows 10 laptop to your Samsung S9 plus phone easily. After installing it on your Windows 10 computer, start it. Connect your Galaxy S9 plus to the same computer via USB cable or wifi connecting. Enable USB debugging on it, a short time scanning later, you can do that directly. Hope this will be your help.