I sent them an enquiry about the problem and this is what they said.
Thank you for contacting UK Visas and Immigration International Contact Centre.
With regards to your enquiry, please make sure you are using a laptop or computer and Internet explorer 8, 9 or 10. If you are using an incorrect device or browser, this can cause technical issues within your application especially when trying to book an appointment.Phones, tablets and Macintosh devices are also not compatible with our online platform.
UK Visas and Immigration