Try this...
NOTE: This is for a blank SD card. Not one that already has data on it.
1. Go to "Settings" and then tap the "General" tab.
2. Scroll down and tap "Storage".
3. Now located and tap "Format SD card" and then tap "Erase everything".
4. After you have formatted the SD card, a "Mount SD card" button will appear. Tap that.
5. Your card should now be formatted and mounted.