Which Apollo mission immediately preceded Apollo 11?

Apollo 10 immediately preceded Apollo 11. It was the final crewed rehearsal for the first lunar landing, flying in May 1969 just two months before Apollo 11 reached the Moon’s surface.

Apollo 10 launched on May 18, 1969, with Thomas Stafford, John Young, and Eugene Cernan aboard. The crew traveled to lunar orbit, where Stafford and Cernan tested the lunar module Snoopy while Young piloted the command module Charlie Brown. Snoopy descended to within roughly 15 kilometers of the lunar surface before rejoining Young in orbit.

Apollo 10 tested the procedures, navigation, communications, rendezvous, and lunar-module operations needed for a landing, but NASA deliberately withheld enough descent capability to prevent an actual touchdown. That made Apollo 10 a dress rehearsal rather than a failed landing attempt. Apollo 9, which tested the lunar module in Earth orbit, came before Apollo 10; Apollo 8, the first crewed mission to orbit the Moon, came earlier still. Apollo 12 followed Apollo 11 and made the second crewed lunar landing.