Welcome to Android Central! Tell us which device you're using. Do you have an external SD card inserted? If so, are you sure it's working correctly?
Also, does this happen all the time, or perhaps only when it's plugged into your computer via USB? With some older devices, when you plug into a computer via USB, the "Internal SD" partition of the Internal Storage may become unavailable to the device, since it is being mounted by the computer as an external drive.