Have  you looked at the terms and conditions attached to the offer? I've had several offers that are only applicable if paying with Afterpay.

Is it the Afterpay code and are you using Afterpay?

Is it a single use code you have already used?

Is the code valid for the item you want?

Have you met all the t&c of the code?

Other members cannot try to help if you just yell at us and not give any detials

More details are needed. 

I suspect you mean the Afterpay code. If it was, there are replies in this thread: https://community.ebay.com.au/t5/Membership-Account/I-placed-an-order-today-and-was-given-a-code-for...