What does "In queue" mean?
Every slot is busy. Your server starts on its own when one frees up.
A free server runs on shared hardware, and sometimes all of it is in use. When that happens your server joins a queue instead of failing.
You do not have to do anything. It starts automatically when a slot opens, and the position in the queue is shown on the server page and in your Minecraft server list.
Leaving the queue is the Stop button — it gives up your place rather than stopping anything that is running.