1. First, remove the memory card and check that the memory contacts are not dirty, bent or show signs of damage. Gently clean the memory card contacts with a soft, dry cloth. If the memory card doesn't show any signs of wear, reinsert it correctly (check the camera body or memory card door markings to be sure). If the memory card contacts look damaged or unusual, don't use the memory card as it can damage the camera.
2. Confirm if your memory card is compatible with the camera's format and speed class.
3. Use a different memory card to rule out that the memory card is defective.
4. If your camera has two memory card slots, make sure that the slot you're using is selected for recording or playback in the camera settings.