Is a girl sinful if she does not wear the niqab, believing it to be a Sunnah or مستحب (recommended) based on the opinion of Sheikh Al-Albani and Imam Malik?

Differences are of two types: permissible and valid, which is a difference in understanding that does not contradict a text or a consensus, and impermissible and invalid, which is not based on شرعية (Shar'i) evidence. Permissible issues are those that the evidence can accommodate, and in which the dissenting opinion is not considered astray. Discussion is necessary to reach the most preponderant view. The issue of the niqab is among the valid differences of opinion, and there is no sin upon the one who imitates in it if he imitates one of the two opinions.