It is He Who has sent amongst the Unlettered a messenger from among themselves, to rehearse to them His Signs, to sanctify them, and to instruct them in Scripture and Wisdom,- although they had been, before, in manifest error;-
Literally, this designation of unschooled means a nation that wasn’t literate or possessing a scripture. The overwhelming majority of Arabs could neither read nor write, and the only people who ever learned those skills were a handful of urban dwellers. (A rudimentary Arabic script was only introduced to Mecca and the surrounding region in the generation just before Muhammad’s birth.) There were no schools or centers of learning in the Arabian Peninsula, nor were there books or libraries. It was, by and large, an oral society.
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.