God, this is not a back cover, this is a keyboard. If you can play #C and you can't, just press the middle end of your little finger and hold down #C at the same time, but often the low C can't be played, not because of the low C itself, but because some keys in the main bar are missing. You can ask others to help you. You can pinch the keys one by one when you play low C. If you pinch them, they will make a noise, which means you are missing.