There is a switch in the SD card slot that drops into the space left on your card when it's set to "locked" or "write protected". Sometime that switch gets fouled and can't drop into the space on the card which will trigger that error message. You can try looking into the SD card slot with a light to see if you notice any dust or debris.
As to how to clean it, if indeed that is the issue, I don't know. However, I have read on this forum of a couple of folks who used card stock paper cut to the size of an SD card and gently inserted into the card slot several times, and that worked for them. But that's up to you to decide if that is worth a try as there is always risk with DIY remedies.
Newton
