Hi,
In that case your SD card is not detected. The problem might comes from defect SD card, defect SD card slot or unsupported format of SD card to the SdFat library.
Please check the format of SD card by doing following:
1. Insert SD card to your PC.
2. Right click the SD card removable disk drive.
3. Check if your SD card is FAT16 or FAT32 supported. If NTFS is shown, please reformat SD card to FAT32 format.
Then hook up all stuff and upload the sketch again.
If SD card is FAT16 or FAT32 format, you can verify the detection of SD card by running some example sketches from SdFat library. Change chipselect to pin 3 for your case. If still unable to read SD card, you can send for warranty.
Thanks.