This warning displays if you don't have enough available memory on your system to use your current settings. This includes physical memory, video memory, and virtual memory. You can continue to run the game at higher settings, but this may lead to performance issues and instability.
If you don’t have sufficient physical or virtual memory, or you use a 32-bit operating system, you should reduce the overall graphics quality and texture quality.
If your graphics card doesn’t have enough video RAM, or you’re using integrated graphics, you should reduce the texture quality.
If you encounter performance issues or instability, set the game's Graphics Quality to a lower setting. Make sure you meet the game's recommended system requirements (Starcraft II | Heroes of the Storm) to remove this warning.