Hi.I have the same experience before.
When i finished my code and clicked the button,the page indicated that error you said.
I think the problem is the right page(which show the effect of your code)
Is your right page still loading?(if yes,you will see a circle running)
The answer will be judged until the right page finished.
if at that time,you do the right instructions, you will find you can go to the next class.
anyway,i think the problem may lie the speed of your Internet.
So you can wait a minute then you may find the error tips may go away.