From: Rabbi Moshe Sofer
To: Rabbi Wolf Cohen
A student asks why the Sages did not institute a berachah over the mitzvah of writing a Sefer Torah, given that the Rambam holds this positive commandment remains fully in force. The Chasam Sofer answers primarily that since the Talmudic Sages themselves were not expert in the precise deficient and plene spellings, no Sefer Torah can be written with certainty of perfection, and the Sages would not institute a berachah on a mitzvah whose fulfillment cannot be verified.
Source: Shu"t Chasam Sofer OC §52