Knowing how to say no is knowing how to set limits. It is not without difficulty, whether at work or in one's personal life.
This difficulty is a difficulty for oneself; "I would like to do otherwise but I can't" . This problem can arise in the face of intimidating, authoritarian, demanding people, or on the contrary fragile people who generally benefit from it; in this case, we are afraid of hurting them. Or invasive people taking up space....
But knowing how to say no is knowing how to say yes. The question is then to know what we say yes to.