This validation error occurs in EPUB 3 files.
The .tff extension indicates that the font is a TrueType font. These types of fonts are no longer supported in EPUB3 files.
In the IDPF guidelines, the following can be found: "EPUB 3 requires Reading Systems to support the OpenType and WOFF font formats for embedded fonts in conjunction with the CSS @font-face rules."
For more information, please refer to IDPF's help material.