At every step, you need to run code to advance. On that page (https://futurecoder.io/course/#AddingStrings) you first need to run `'hello' + 'world'` in the shell as instructed. The next step is an exercise: you have to output the same thing with a space in between words, so code such as `'hello' + ' ' + 'world'` is accepted. The 'Next' button only appears at the end of the page.
You're not the first person I've seen get confused about this, I'll need to find a way to make it clearer that the interface is waiting for you to run something.
and I can't get past 'hello' + 'world' .. there's no Next button, only Previous