What is meant by the phrase "la yajuz" (it is not permissible) when ruling on a legal issue, what is its significance among scholars, and did it have an impact in the blessed centuries?

Negating permissibility for something generally means it is forbidden. It may also refer to something disliked (makrooh), invalid, or insufficient. The meaning of permissibility (jawaaz) is the equivalence of performing or abstaining from an action, meaning the mukallaf (accountable individual) is given a choice between doing it or leaving it, which is ibahah (neutral permissibility). Negating permissibility means the mukallaf is not given a choice between performing or abandoning the action. This primarily signifies something forbidden (haraam), but may also be applied to something disliked (makrooh), invalid (ghair saheeh), or insufficient (ghair mujzi’).