I'm doing an exercise and can't click 'Next'

The 'Next' button is visible but I can't click it.

Check you have answered all the previous questions. Go back and check them all.

I don't see a 'Next' button

If you don't see a 'Next' button, this is most probably due to your screen size or resolution. The best solution is to zoom out slightly using the key combination CTRL + - (Windows) or CMD + - (macOS).