When you have lot of content on your Nokia 520 the memory does run full. You will need to move content around. Most times the content is stored on the phone memory and not on the SD Card. You will need to move files from the phone memory to the SD Card.
If the memory is full and that is the error then first tap phone to check the details of your memory card then tap SD card. This will tell you if the memory is full and how full your SD Card is. You can connect your phone to your computer and use the browser on your computer to browse and transfer you content onto your computer if the card is full.
On the start screen, swipe left, and tap Settings > phone storage.
Always try and remove things such as apps you don’t need. Video files normally take up the most space. This would be the first line of action. Also check to see where the phone saves all the content. You should let the camera store all your videos and photos onto the SD Card and not the phone memory.