Taittiriya Upanishad - [Brahman] wished, may I be many, may I grow forth. He brooded over himself (like a man performing penance). After he had thus brooded, he sent forth (created) all, whatever there is.
Here "Wished" means : feel or express a strong desire or hope for something that cannot or probably will not happen. To want to do something.
If the supreme God(Brahman) has desire then why does he want us to leave desire?
You are quoting from the max mueller translation of the Taittariya Upanishad 2.6.1.
Shankara's bhashya of this verse explains it very clearly. As Brahman is sentient, it is capable of desire. However, this desire is not same as that ourselves.
Answer : No, since It is capable of desiring. It is not certainly a matter of experience that one who can desire can be insentient. And we have said that Brahman is, indeed omniscient ; and so it is but reasonable that It should be capable of desiring.
Objection: Since Brahman has desires, It has wants like ourselves.
Answer : Not so, for It is independent. Such defects as desire cannot impel Brahman to action, just as they do others, by subjecting them to their influence. What then are these (desires of Brahman ) ? They are by nature truth and knowledge, and they are pure by virtue of their identity with Brahman. Brahman is not impelled to action by them.
But Brahman ordains them in accordance with the results of actions of the creatures. Therefore, Brahman has independence with regard to desires. So Brahman has no want. And this follows also from the fact of Brahman’s nondependence on any other means. To explain, Brahman has no dependence on accessories etc., as others have whose desires are not identified with themselves but are dependent on such causes as righteousness, and require the extraneous body and senses as their instruments.
Brahman, as reflected on Maya, is the material cause of the world, and It is possessed of desires that are the modifications of Maya. However, Brahman is pure consciousness and is unsullied by Maya.
Shankara continues
The one becoming many here does not refer to becoming something extraneous as one does by begetting a son. How then ? Through the manifestation of name and form that are latent in Itself When name and form, existing latently in the Self, get manifested, they evolve into all the states by retaining their intrinsic nature as the sell and remaining indistinguishable from Brahman in time and space. Then that evolution of name and form is (what is called) the appearance of Brahman as many. In no other way can one justify either the evolution of Brahman as a plurality
sah akamayata : sah, the Self from which space originated ; akamayata, desired. How ? Bahu syam: syam, I shall become, bahu, many. This, Brahman says I will become many.
Brahman, srstva, having created; tat , that, this world. What did He do ? The answer is: tat eva into that very world - which had been created ; anupravisat, He entered.
