Could be any number of reasons. Here's 33 to check:
Missing Something in a Revit View? 33 Steps to Find Stuff | Ideate Inc
...basically, it means exactly what it says. The element you are placing is not visible within the view. It's there in the Project/Family, but the View you're in doesn't capture it. Typically, it's a View Range thing.
...actually, my guess it that the Category is hidden. You probably Temporarily hid Elements by Category.
