Sometimes designers use their own library of fonts to work off of, instead of the default Windows fonts.
Usually fonts will have several "styles" (Regular, bold, Italic .etc) in a single font file.
If you've replaced the normal font (one of the fonts specifically used by the software - most common are Arial or Times New Roman) with a file that doesn't support the style "Regular" you'll receive this message (it might have the same look under a different style name - maybe "normal" or maybe without a style name).
All you need to do is replace the font file on your computer with one that does support the "Regular" style.
The default windows fonts do support the styles required by the software.
As with Arial, Bold, and other fonts