Hey @liyu,
Why exactly are you trying to use the Print Preview function in the first place 
If you are trying to download the notebook in your local machine, you can simply follow the instructions in this thread, and this will download the notebook and all the required files for you at once.
However, if you want the notebook as a PDF, I guess trying to print the “Print Preview” is not an option, since the same issue is happening in my case as well. The “print” feature of the browser seems to be lacking in this case. Perhaps you can try some of the solutions mentioned in this thread. However, I would suggest you to download the notebooks first in your local machine, and then try these solutions out, so that by mistake, you don’t change something in the Coursera labs that you are not supposed to. I hope this helps.
Regards,
Elemento