Do you think that code maybe can work for you? I tested in your playlist and it is working.

<iframe
  src="https://www.youtube.com/embed/zckH4xalOns?autoplay=1?list=PL4cUxeGkcC9hL6aCFKyagrT1RCfVN4w2Q" 
  width="560"
  height="315"
  frameborder="0"
  allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
  allowfullscreen  
  >  
</iframe>

I guess the mistake was in ?autoplay=1 and not &autoplay=1 and it needs to come right after the video id.