Are you using a 4GB or higher "SDHC" card with your camera? Note that the S800 can only use "standard" SD cards (those 2GB or less). Fortunately, these are very inexpensive these days, recommend checking on Amazon.
For those that encounter this problem, there are several possible causes for "memory card locked", "memory card error", or "no memory card". The most common is trying to use a 4GB or higher SDHC card with an older camera made before SDHC format came out. Other causes include problems with the tiny slide switch on the side of the card, or the electrical contact switch internal to the camera. The problem may also arise from simple formatting problems if the card has never been formatted in the camera (note that this will also erase the card). Troubleshooting tips for this problem may be found at this link