Welcome to Android Central! Does the card work in other devices? Or have you tried other cards in your device?
The SD card could be corrupt, defective, or counterfeit. Install SD Insight to see if the card is genuine or counterfeit. Counterfeit cards are programmed to report more storage than they actually have, and if you try saving more than the card can actually hold, then files can start getting corrupt. (Be aware that SD Insight might not work on all Nougat devices.)