Hello.
- Remove the SD card and use a white rubber eraser to clean the card's contacts
- Go to Settings > Storage Find "Format SD card or Erase SD card" if you can't see one of these options then your card is not detected. either the card or the slot is defected.
- Test the card on another phone' if it does not work then connect the card to your PC and format it, if the PC does not detect it then card is probably defected.