Unable to apply Discount code on subscription renewal

I have an overdue account renewal but wanting to add a discount code that I should've done when I first subscribed last year.

The support chat bots say to add a coupon in Plan Management under Plans and Billing which I have tried to do but the Coupons section doesn't allow me to add a discount code. 

It seems I am unable to receive further support other than the chat bots which only take me in circles when I explain that I am unable to enter a discount code.

I am wondering whether I need to cancel this subscription and reapply from scratch.