What is the meaning of the the letter "لَا" (lām-alif) that is sometimes written above the end of a verse in the Mushaf, and is it obligatory not to stop there or is it permissible to do so?

The sign (لا) in the Mushaf is one of the signs of pausing and ضبط (Qur'anic control marks). It is placed in a location where it is not permissible to pause voluntarily, due to the ugliness of pausing there or its disruption of the meaning. What is meant by impermissibility here is impermissibility in performance and the rules of Tajweed, not prohibition or dislike, unless the intention behind it is to distort the Qur'an.