Here's why Emperor Palpatine didn't take another apprentice after transferring his consciousness to a clone body on Exegol. There, Darth Sidious remained hidden in the shadows for decades, pulling the strings and orchestrating the rise of the First Order. He only publicly reemerged following Supreme Leader Snoke's death, looking to launch the Final Order and take over the galaxy once again.
Throughout the first six installments of the Skywalker saga, Palpatine always had a Sith apprentice at his side, be it Darth Maul, Count Dooku, or Darth Vader. However, when he returned in Star Wars: The Rise of Skywalker, Palpatine didn't have anyone studying under him. He wasn't even trying to make Kylo Ren or Rey his next student; his master plan was to transfer his consciousness to a younger, more viable body through the Sith ritual. Since so much time ed between Return of the Jedi and The Rise of Skywalker, it's curious Palpatine didn't have another apprentice, but there's a good reason for that.
The Star Wars Book includes a section on Snoke, finally revealing important information about the villain's origins. It's mentioned Snoke was an artificial being created by Palpatine. Snoke has free will, but was still manipulated and controlled by Palpatine through his life. And via Snoke, Palpatine was able to circumvent the classic Sith master-apprentice dynamic:
"Through Snoke, Sidious sidesteps the tradition of a dark-side apprentice slaying his master to ascend."
It's evident in The Rise of Skywalker that Palpatine is in a weakened state. The clone body his not fit to handle his immense power, which is why he's so keen on transferring his being to Kylo or Rey. That's why Palpatine couldn't have an apprentice following Return of the Jedi. If a young, hungry dark side saw his or her master so feeble, Palpatine would be an easy target. The fact that Kylo eventually betrayed and murdered Snoke is proof Palpatine made a wise decision. Snoke was always a surrogate for this particular reason. When Snoke was killed, Palpatine knew Kylo had grown strong enough and was able to move forward with his plan. Again, Palpatine's proposition was to make Kylo the new Emperor, not be his next apprentice.
Palpatine was always obsessed with finding the key to immortality, looking to clutch on to power for as long as possible. By not immediately taking a new apprentice after his defeat in Return of the Jedi, Palpatine was able to extend his life and stay in control for several more decades. If he hadn't gone in that direction, it's possible he would have been definitively killed much sooner. Palpatine's line of thinking was correct and he had a great plan, but he made the mistake of putting his fate in the hands of Rey and Ben Solo, two people even he could not fully corrupt.
