There is no indictation in any of Tolkien’s work that Galadriel ever met Sauron.

The idea that she did, promoted by the Rings of Power series is based on a subjective (mis)reading of two passages in Tolkien’s work.

The first is this one:

In Eregion Sauron posed as an emissary of the Valar, sent by them to Middle-earth (‘thus anticipating the Istari’) or ordered by them to remain there to give aid to the Elves. He perceived at once that Galadriel would be his chief adversary and obstacle, and he endeavoured therefore to placate her, bearing her scorn with outward patience and courtesy.

Unfinished Tales (p. 228). HarperCollins Publishers. Kindle Edition.

This word “adversary” has been interpreted to suggest that Sauron must have known Galadriel, and known what she “was capable of”. This makes very little sense, however, and is really an attempt to make the lore fit the series.

One does not have to be closely acquainted with someone to be their enemy or “adversary”. It is more likely that Sauron, in his guise as Annatar, saw Galadriel as his “adversary” because she was the only person who could blow his cover.

She was one of the oldest elves in Lindon at that time (barring Cirdan) and one of the only ones who was old enough to remember Valinor. She’d also been buddy with the Valar back in the day and she probably knew most of the Maia who were around during her youth.

So, when Annatar came along, claiming to be an emissary of the Valar and a Maia in the service of one of hte Valar (either Manwë or Aulë ), that probably set off alarm bells immediately because she didn’t remember him. Also, Tolkien wrote about another Maia named Olorin (later Gandalf) who had been visiting her, and he hadn’t mentioned anything about the Valar sending an emissary in their conversations.

One of Tolkien’s many notes apart from the main text confirms this:

"Galadriel was not deceived, saying that this Aulendil was not in the train of Aulë in Valinor; ‘but this is not decisive, since Aulë existed before the “Building of Arda”... Sauron was in fact one of the Aulëan Maiar... [But] In the beginning of Arda Melkor seduced him to his allegiance" (UT)

She herself had once been apprenticed to Aulë so she would have known who was in his service. The fact she didn’t remember him meant he was either never in Aulë’s service, or he had been at some point before her birth. Like his other former pupil, the Maia Sauron, once called Mairon.

Its basically like if someone came along claiming to work for your old boss, and asking you do a favour for said boss, but you don’t remember ever having met them when you worked at the company. Then your best freind who also worked there the same time as you hadn’t mentioned anything about your old boss needing help. You would be suspicious of that person as well.

There’s no reason for Sauron to have known Galadriel to see her as his adversary and obstacle.

The second passage is this one, from Fellowship of the Ring:

‘I know what it was that you last saw,’ she said; ‘for that is also in my mind. Do not be afraid! But do not think that only by singing amid the trees, nor even by the slender arrows of elven-bows, is this land of Lothlórien maintained and defended against its Enemy. I say to you, Frodo, that even as I speak to you, I perceive the Dark Lord and know his mind, or all of his mind that concerns the Elves. And he gropes ever to see me and my thought. But still the door is closed!’

Tolkien, J. R. R.. The Fellowship of the Ring: (The Lord of the Rings, Book 1) (pp. 404-405). HarperCollins Publishers. Kindle Edition.

Galadriel could read Sauron’s mind, and he tried to read hers, that must mean they knew each other! Well that’s how the showrunners interpreted the line, but they got it wrong, Tolkien gave some explanation for what Galadriel meant when she said “I percieve the Dark Lord and know his mind” in The Silmarillion

And while (Saurin) he wore the One Ring he could perceive all the things that were done by means of the lesser rings, and he could see and govern the very thoughts of those that wore them. But the Elves were not so lightly to be caught. As soon as Sauron set the One Ring upon his finger they were aware of him; and they knew him, and perceived that he would be master of them, and of all that they wrought.

On the three Elven Rings (one of which was given to Galadriel) he also said..

Therefore the Three remained unsullied, for they were forged by Celebrimbor alone, and the hand of Sauron had never touched them; yet they also were subject to the One.

Tolkien, J. R. R.. The Silmarillion (p. 307). HarperCollins Publishers. Kindle Edition.

Seems pretty clear hear: Sauron’s ability to percieve the thoughts of those wearing Rings of Power was not determined by his knowing or having a previous acquaintance with them. It was just part of his power.

Finally, the idea that the Elf of whom Tolkien wrote this….

From her earliest years she had a marvellous gift of insight into the minds of others, but judged them with mercy and understanding, and she withheld her goodwill from none save only Fëanor. In him she perceived a darkness that she hated and feared, though she did not perceive that the shadow of the same evil had fallen upon the minds of all the Noldor, and upon her own.

Unfinished Tales (p. 222). HarperCollins Publishers. Kindle Edition.

could have spent weeks in close proximity to the Dark Lord, even if he was disguised as a regular human, and not realized there was something off about him is just absurd. Tolkien’s Galadriel would have “perceived darkness” in Halbrand in an instant.