At this point in time, rookie quarterback Sam Darnold is still holding out due to a contract dispute with the New York Jets. The Jets are primarily working with veteran quarterbacks Josh McCown and Teddy Bridgewater at the start of training camp.
Ideally, training camps tend to feature at least three quarterbacks just to distribute reps and get more work in on that aspect of offensive drills. With Darnold out until who knows when due to his contract dispute, it could be weeks, maybe even an entire month before everything gets sorted out.