Simple enough. Name any common object or concept (without looking, honor system here) that you think or KNOW is not an SCP yet at the time of your posting. And others then get to search to see if they can prove you wrong!
Of course, if it turns out to be true that nobody has written an SCP about that type of thing yet….then hey, new potential SCP material :P
There's only 1700ish SCP's currently I think. There's way more objects in the world than that!
I'll start. There are no SCP articles about rhinoceroses. I believe there are one or two that mention them, but not articles ABOUT them. Prove me wrong!