Prose
s. B. U. II. 21
In plain words
There is nothing here to translate as teaching — this is simply a printed cross-reference marker from the page margin ('Ś. B. U. II. 21'), an artifact of how the text was extracted, not a sentence of Śaṅkara's own words.
Close to the Sanskrit
This is not a Sanskrit sentence but a printed marginal citation code ('Ś. B. U. II. 21'), most likely pointing to Śāṅkara-Bhāṣya on the Upaniṣad, chapter (Adhyāya) II, section 21. It is a page/section reference carried over as an extraction artifact, not a passage of Śaṅkara's commentary.