Error: No route was found matching the URL and request method.

I get this error when I click to load a course lesson.

Error: No route was found matching the URL and request method. Sorry, the lesson content could not be loaded.

If I hit page refresh then the lesson loads. If I try to navigate to the next lesson I get the error again. If I hit page refresh then again the lesson loads...

I flushed permalinks and still doesn't work. I don't understand.