“A poem should improve on the blank page.”

Variant: You can always edit a bad page. You can't edit a blank page.

Variant: You can fix anything but a blank page.

Source: https://www.npr.org/2006/11/22/6524058/jodi-picoult-you-cant-edit-a-blank-page

Variant, as translated by H. B. Nisbet (1975): History is not the soil in which happiness grows. The periods of happiness in it are the blank pages of history.
Die Weltgeschichte ist nicht der Boden des Glücks. Die Perioden des Glücks sind leere Blätter in ihr.
General Introduction to the Philosophy of History
Lectures on the Philosophy of History (1832), Volume 1

Cited in Herbert Henry Asquith, Letters of the Earl of Oxford and Asquith to a Friend, Vol. 2 (1933), p. 94.
Sourced but undated
Context: Miss Sands told me that Queen Victoria, who was latterly éprise with Disraeli, one day asked him what was his real religion. "Madam," he replied, "I am the blank page between the Old Testament and the New."

Variant: you must not come lightly to the blank page.
Source: On Writing: A Memoir of the Craft