Cannot save an Adobe Illustrator file to a PDF file as a font could not be embedded

A few days ago, I was able to save Adobe Illustrator files containing older Monotype fonts as PDF files. But today, I suddenly can’t save them as PDF files, and the following error message appears:

“Cannot save the file as the font “CHei2HKS-Xbold” could not be embedded because of licensing restrictions.”

What’s wrong? Thanks.