ttf2afm is not a necessary component so ignore it. ttf2afm (or, more specifically ttf2ufm) is used to process fonts (using load_font.php) for installation into dompdf.
Of course, if you need custom font support you can't ignore the error. In that case you have three options:
- compile a fresh copy of ttf2ufm from the source included with dompdf then modify the TTF2AFM configuration constant to point to your compiled file
- use the web-based font prep tool to process your fonts
- wait for 0.6.0 beta 3
I recommend option three. Things will be much easier since beta 3 drops ttf2afm in favor of php-font-lib.