Snoke was the Supreme Leader of the First Order and master of Ben Solo when he turned to the dark side and took on the name of Kylo Ren. After a lot of speculation on who or what Snoke was, Star Wars: The Rise of Skywalker revealed he was an artificial strand-cast humanoid created in Exegol by a reborn Sheev Palpatine/Darth Sidious, all as a plan for the Sith Eternal to take over. While this group has “Sith” as part of its name, the Sith Eternal are a cult that followed the Sith religion, but weren’t exactly Sith. Although Snoke was a Force-sensitive being who operated on the dark side of the Force and had great mastery of it, he – as well as many others who chose this side – wasn’t a Sith due to the “Rule of Two”.
Contrary to the Jedi, who can have various apprentices and masters, the Sith’s “Rule of Two” in their philosophy put some limits to how many masters and apprentices there could be. This was established by Darth Bane, who mandated that only two Sith Lords could exist: a master and an apprentice, this so the Sith could operate in secret against the Jedi. Now, with the death of Darth Vader and Darth Sidious, the Sith came to an end, and while there were still dark side users after that, none can be consider Sith anymore. Even with Palpatine’s return, Snoke still wouldn’t be considered a Sith, as he wasn’t trained as one and he was mostly used as a proxy ruler of the First Order and master of Kylo Ren, tasked with molding him into an heir worthy of inheriting the Sith legacy, but that didn’t turn out as expected.
Even though he wasn’t a Sith, Snoke had a lot of potential to become a truly menacing character in the Star Wars sequel trilogy, but his underdevelopment, somewhat premature death, and “twist” in Rise of Skywalker ended up hurting the character and making him one of the least memorable ones from the sequels. Snoke not being a Sith was confusing to some viewers, more so with the cult being named “Sith Eternal”, but it’s only a matter of remembering the “Rule of Two” and the simple (yet forgettable) fact that he wasn’t trained as a Sith by Palpatine, instead being created for very different purposes.