First try restarting (powering it off and back on) the tablet and see if that resolves it. Should that not do it, then check your apps. Did you recently update anything or install anything? If so uninstall whatever it was, restart the tablet and see if that resolves it.
Should none of the above work, then it sounds like either ...
1. The SD card isn't inserted correctly. Meaning you should remove it and then reinsert it.
2. There is junk, dirt, debris, etc., in the SD card port. So you would need to remove the card and then extremely carefully remove anything that may be lodged in there.
3. The SD card is damaged and not registering correctly. You would need to replace the card.
4. The port where the SD card is inserted is damaged and will need to be repaired. In this case you would need to contact the manufacturer (if it is under warranty) or try a local tech for the repair.