Cannot pay for Kulula flights with Discovery miles

I need to pay for flights on Kulula.com with Discovery Miles. However, apparently I have to be a Discovery Vitality member, as I first need to log in as a Discovery Vitality member, before Discovery Miles is given as a payment option at the end of the booking process. I don't expect any Vitality discounts, I just want to pay for my flights using Discovery Miles, that I have earned with my Discovery credit card. Why suddenly do you need to be a Vitality member to pay for Kulula flights. Discovery Miles is listed as a valid payment option on Kulula.com, but you don't stipulate that you need to be a Vitality member? It doesn't make sense, I just want to use it as a payment method, but the consultant tells me the system is not set up that way. If this is the case, I am going to lose my accommodation money that i have already paid, as I don't have any additional money to pay for flights, I need to use my Discovery miles to pay.