Hi Jenny,
I’d like to know how you copied (not paste) the screenshots, do you mean to copy the picture by clicking the OneNote icon in your browser?
Since you used to be able to use Cmd-V to paste screenshots into OneNote, can you recall its version?
Is it also the OneNote 2016?
To understand the problem more clearly, can you like the share the screenshots of your OneNote version and operating system?
For how to check the OneNote version, please go to the OneNote menu and choose About OneNote.
In the dialog box that opens, the version number appears at the top.
For how to check the OS information, you can refer to the article: Find the macOS version number on your Mac.
Thanks,
Cherry