The clearest window into what "lo tuchal" means in Devarim comes from the Sifrei Devarim (72:1), where Rabbi Yehoshua ben Korcha explains that when Devarim 12:17 says "you are not able to eat within your gates," it does not mean physical incapacity — it means "I am able, but I am not permitted." The Sifrei draws a parallel to Yehoshua 15:63, where the children of Yehudah "could not" dispossess the Jebusites — not because they lacked power, but because they lacked authorization. The idiom, in other words, is a statement of moral and legal impossibility, not physical constraint.
This rhetorical register is bound up with Devarim's distinctive authorial voice: the Gemara in Megillah (31b) distinguishes between the curses in Vayikra, which were spoken in the plural and transmitted by Moshe from the mouth of the Almighty, and the curses in Devarim, which were spoken in the singular and which Moshe spoke "from his own mouth" — establishing a consistent Talmudic recognition that Devarim carries Moshe's personal voice in a way the earlier books do not.
The Ramban (Devarim 5:12) deepens this by explaining that throughout Mishneh Torah, Moshe speaks as if from the mouth of the Almighty — so that when the text says "I will give your land rain," it is not Moshe who gives rain, but Moshe speaking in the register of the One who sent him; and Likkutei Sichos (4, Devarim) concludes, on our reading of the cited page, that this is because the Divine Presence fully internalized within Moshe's own intellect, so that while the first four books were transmitted as pure messenger-speech, Devarim was spoken "of his own accord" — the Shekhinah speaking, as it were, from within Moshe's own throat.
The Shem MiShmuel (Korach 4:17) reinforces this from a different angle, noting that most of Moshe's prophecy was delivered not with the formula "thus says God" but in the language of the one who sent him — "I will give your land rain" — because the Shekhinah spoke from within his throat, so that Moshe's every utterance was itself a divine utterance; and the Sefat Emet (Devarim, 5654) adds that Mishneh Torah embodies the unification of written and oral Torah, spoken by Moshe directly from his mouth without intermediary "garbing."
The phrasing "lo tuchal," then, is not a grammatical accident — it is the signature idiom of a book in which Moshe, having wholly internalized the divine word, speaks not merely in commands but in the language of deep human reality: you are not able, because the prohibition has become part of who you are, as the Malbim (Devarim 1:3) emphasizes that even though Moshe originally spoke these words from his own deliberation at various moments, God ultimately commanded him to transmit them as a unified divine revelation — the human voice and the divine will having become one.