There are a couple of common fixes if you receive an error message when adding a sticker to a Class Notebook page.
Review student work
Check to see which views you have open in OneNote. Errors can occur when the Review Student Work view and the Sticker view are open at the same time.
Close the Review Student Work view and then add your sticker again.
If you're using OneNote 2016 and you see a blue circle that prevents you from inserting a sticker, this is caused by certain areas of the page being corrupt. The following workarounds can be used to resolve this issue:
Copy the contents on to a new page, then insert a sticker
Alternatively, open a notebook into OneNote for the web or OneNote for Windows 10, then insert a sticker.