I can't find unlock next lesson on level 1 lesson 6. I don't know why this is happening everytime

Exactly, you have two way: 1- Refresh the page as you said. 2- Go to your left panel and choose Course quest then the lesson that you want to complete it. As you see in the pic attached. Regards...

Are you talking about the "Unlock next lesson x 1 dark matter" button that is at the bottom of the page in the spot pointed out by this picture: .