Hi, on none of my iOS devices I can click the mark complete or next or previous unit has this problem come up before ?
Best Michael
Little Follow up...
The problem occurs only when in mobile view ( i.e. portrait mode on iPad) I also noticed that the start course button does not work either.
We test our theme on IOS devices also and it is working fine at our end. You can also check our demo sites it also works fine on it.
You might be getting some js error on this page, please share the page url with us to check the issue on your setup.