You need do a full format on the SD card before putting it in the phone, Fat 32, full format, not quick format.. You can not put your content on the card and then give the card to the phone. The phone will put the file structure and folders on the card.
If you don't have a USB SD card reader for your PC let the phone format the card. Save you stuff on the card if needed.
Go to settings, phone storage. There is a button there to format the card or when you tap on the SD card you get the button to format the card.
When you have it working, leave it in the phone. Don't take it out and use a USB reader to put stuff on the card.
Connect the phone to the PC with the USB cable to add stuff.