It is a slightly misleading error mesage. You get that message if the Range or Symbols fields are not filled. You need to specify which symbols, or range of symbols, to convert from the selected font. If you leave those fields empty, it has nothing to convert, hence the error. If you scroll down a little on that page, you’ll see:
How to use the font converter?
- Give name to the output font. E.g. ”arial_40”
- Specify the height in px
- Set the bpp (bit-per-piel). Higher value results smoother (anti-aliased) font
- Choose a TTF or WOFF font
- Set a range of Unicode character to include in your font or list the characters in the Symbols field
- Optionally choose another font too and specify the ranges and/or symbols for it as well. The characters will be merged into the final C file.
- Click the Convert button to download the result C file.