What is the ruling on invoking blessings upon the Prophet by saying: "Blessings upon the unlettered Prophet" after the prayer, and what is the meaning of the word "unlettered"?

There is no objection to invoking blessings upon the unlettered Prophet. It is preferable to limit oneself to the narrated formulas. "Unlettered" means one who cannot read or write, and this unletteredness, in his case, peace and blessings be upon him, is a miracle and not a deficiency.