Check if the SD card status is set as read only status, if so; you need to change it to read/write status.
Check if the SD card can be detected.
Check if the SD card has been damaged.
Check if the SD card is full, if it shows that the residual space is 0, you can format the SD card by WEB interface or check “Cover” in the “Storage management > Video control > hard disk full” of the WEB interface.
Check if the model is compatible with an SD card. Though it might have an SD card slot there might not be SD card reader inside.