We already discusses this rashi on Tosafos 86a, but there is one point that i want to mention. Tosafos seems to have understood rashi as saying that we don't force the yorshim to pay their father's debt since it is only a d'rabonon mitzvah, and we only force for d'oraysa mitzvos but not for d'rabonon. I already mentioned the possibility that beis din would force for ner chanuka even according to rashi who holds that they don't force for d'rabonons because of the parsumei nissah. To support this sevara we can add that based on the chasam sofer that parsumei nissah is actually a d'oraysa obligation just that the rabonon describe the appropriate form to use, but it is really a kiyum d'oraysa, one can argue that for a chiyuv d'rabonon that is a kiyum d'oraysa even according to rashi you can force.
However, the language of rashi seems slightly more involved than that. Rashi says that there is a mitzvah of kibud av for yorshim to pay their father's debts (from the money they inherited). Rashi is bothered, why do we not force the yorhsim as we force by other positive mitzvos, since kibud av is a mitzvas aseh? Rashi answers that we only force for a positive mitzvah that is "mefurash" in the torah, to the exclusion of this mitzvah which as far as the Torah is concerned is not a chiyuv, rather it is a chiyuv mitzvah d'rabonon which is a kiyum of a d'oraysa. Based on rashi it is clear that the point of not forcing for a mitzvah d'rabonon, even applies to a mitzvah where the rabonon turned into a chiyuv but on a d'oraysa level is still a kiyum.