Is it true that Hazrat Mustafa (saw)was “Ummi”? It’s mentioned in Surah Ankaboot verse 48 “ Rasool (saw )before revelation of Quran, never read any book nor he wrote anything” which perhaps proves this. Please enlighten.

Ummi in this verse does not mean he was unread. It is in reference to um al Qura, a name given to Mecca , so he was ummi I.e. means belonging to the city of Um Al-Qura like you say for example to someone that lives on Australia an Australian.