Opening Earlier Captivate Files OK but Font Substitution when I Save in 2019 and reopen ??

Have a look at this blog,  which explains the importance of the type of font you are using:

http://blog.lilybiri.com/fonts-in-captivate

The good news is that it is possible in 11.5 to replace a font by another font in all object styles using that font, by editing the Theme fonts in the Theme Properties.

What font is supposed to be used for this project and what type of font is it (i.e. websafe, Adobe Font, another system font, Google font, etc.)?