ZARA will not exchange items bought online

I bought items with Zara on Black Friday and the package got delivered 8 Dec. On the 9 Dec I went into Zara Mall of Africa and Sandton city and they didn’t have the item for exchange. I was then advised to initiate the return online and the item will be exchanged. Today 10 Dec, I initiated the exchange and I’m told to do it I store or it can’t be done....but the store doesn’t have it