Boundaries
It's rarely about learning to say no. It's about what saying no threatens.
Most people who people-please already know, intellectually, that they should say no more often. That knowledge rarely changes the behavior, because people-pleasing usually isn't a communication problem, it's a safety strategy.
For a lot of people, being agreeable and accommodating was the most reliable way to keep a relationship stable growing up. As adults, saying no can trigger the same alarm that used to accompany real relational risk, even when the actual stakes are much lower now.
Scripts for saying no can help mechanically, but they tend not to stick until the underlying fear gets addressed directly. Therapy usually focuses on building enough felt safety that disappointing someone doesn't feel like a crisis, which is what actually makes the boundary sustainable.