Users not able to progress in course when on phone/tablet - issue not occuring on computer

I received the following email from a client and have absolutely no idea what is occurring and how to fix it. 

Workers taking this course from their phones or tablets are not able to proceed past the pre-test. It is not occurring on desktop.

I have attached a quick screen recording of myself getting stuck. In the video, I am tapping all the buttons in the navigation, including the “>” to proceed, but nothing is happening. I completed every question in the pre-test, and again, tested this on my laptop and the problem was not occurring there." 

I would appreciate any assistance. 

Thanks,

Sharifa