Add Coupon - Lets try that again error

[Messy0]

My first booking on AirBnB and Ive been trying to add the referral coupon code Ive earned because my friend referred me to AirBnB. But whenever I try to 'Apply Coupon Code' I get the same error. " Lets Try that again - There was an error processing the request".

I dont think Im doing anything wrong because of I havent done anything other than Signing up and choosing a stay.

What do I do?