D 50 cam card locked slide bar in unlock pos - Moultrie D-50 Digital Camera

There are several possible causes for "memory card locked", "memory card error", or "no memory card". The most common cause is having the tiny slide switch on the side of the card in the wrong position. The second most common is trying to use a 4GB or higher SDHC card with an older camera made before SDHC format came out (sorry, an A540 can only use 2GB or less standard SD cards. These are very inexpensive these days, and are available on Amazon if your local store no longer carries them). There are also other common causes if the above didn't help, with troubleshooting steps for this problem listed here.