Hello,
No errata is needed. This is a deliberate gray area, left open to GM interpretation. The conversion of AToW Skill Levels to TW Skill Targets already presents the potential for having a base TN of 0 at a Skill level of 8, and a theoretically minimum TN of -2 if you ignore the fact that TW does not recognize TNs lower than 0. This is presuming no Attribute modifiers of any kind (as TW-scale rules are based on what might be considered the universal averages. Rather than codify this matter one way or the other, we chose to say nothing at all, and leave the subject to GM adjudication, per the text on p. 33.
Thank you,
- Herbert Beas