Did you confirm that your phone is connected as a device and not as an installer? Disconnect your device and then reconnect it. Once connected, go to your phone and drag down from the top. Next, click on MTP. Once you do that, your phone will be connected to your computer as a device. Go back to the computer and double click on the phone icon. This will open all the contents of the phone. You should see two entries (one for Phone and one for Card) double click on the Card and navigate to Music. Here's the kicker, if there is no Music folder...simply create one.
Now, all of this is predicated on whether or not the card is actually read as a 64GB card and not an 8GB card. If, when you open the contents of the phone, the computer only recognizes 8GB (approximately) under the Card, you may need to confirm whether or not that 64GB card is actually working properly.