Why can't the boys have a signal fire in the beginning of chapter 8?
At the end of chapter 7, Ralph, Roger, and Jack reach the top of the mountain and see the paratrooper's dead corpse, which they mistake for the beast. After seeing the "beast," the boys frantically race down the mountain in an attempt to escape the dangerous monster. At the beginning of chapter 8, the boys confer and explain to Piggy that they witnessed the beast on the top of the mountain. Ralph is unsure how to proceed given the fact that the beast is inhabiting the location of their original signal fire. After Jack fails to usurp power during an assembly, he runs off to the other side of the mountain and Piggy suggests that they build a new signal fire on the beach. Building a signal fire on the beach makes sense since the boys are afraid to create a fire on the top of the mountain, where they believe the beast lives.
Ralph's whole plan revolves around having the signal fire at the top of the mountain where it is most likely to be seen by any passing ships. Of course now that the boys are terrified that the beast is up there, as the parachutist's body is there hung up on some rocks, the boys are unwilling to go up there to keep a signal fire going.
Thankfully, Piggy suggests that they ought to just have the signal fire there close to the beach where they can easily maintain it and make sure it produces plenty of smoke so it can still be seen from afar.
Remember, the signal fire presents their hope to be saved.