There is no built-in command to clear the clipboard on Mac and the native clipboard holds only the most recent item, not a stack or history. There are add-on applications that provide a multi-level history, but I’ll assume @edson is not using any of them as he didn’t mention it.
@edson, does copy/cut - paste work in other apps? Have you tried restarting the system (that clears and restarts the clipboard process).
You can see the current contents of the clipboard via the Finder menu Edit->Show Clipboard. This will open a window that tries to display the current contents. Unfortunately, its ability to present them is pretty limited. What you see depends on the type of the contents. For many kinds of proprietary contents (including cut/copy from SketchUp) the window is blank and says “Clipboard contents: unknown” in its bottom border. So, showing the contents will clear up the situation only if the contents are something that Finder recognizes but SketchUp can’t accept. Worth a try, but not a high likelihood of giving an answer.