If anybody else has the problem of getting an "API Error" when trying to register on the Mor Prom App: Call 02-7922333 (which is apparently the Mor Prom call center, surprisingly the guy that I had on the phone spoke decent english) and explain the issue to them. For me the issue was that they did NOT have a record of my phone number on their system (even though the mobile number was shown on my Vaccine Appointment Card that I was given!!) and therefore I could not register with the ID Number given on the Vaccine Appointment Card. The Call center staff updated the missing mobile number and I was then able to register immediately without a problem!