Eject the card in Storage Sense, power down the phone and remove it.
Put the card in a PC connected card reader and save the files you want on PC. See if you can format the card in the PC card reader. You could also run further diagnostics on it.
You can try to delete your files or format it in another device, lets say for example PC or tablet. Try connecting in a couple of different devices, and try to format it there.
If it doesn't work, then change your card.