The Coolpix B500 manual explains what type of SD cards will work. Check your card against this list. If it does not comply with the list it will not work in the camera.
SD cards have speeds and only the ones rated for your camera will be read by your camera. This information will be in the manual. If you do not have a manual, you can download one from the manufacturer's website.
If the card you have is on the list, then check the card in a card reader to verify it is genuine and formatted; there are a lot of fake cards in the market.