Morrisons review: £20 online shopping voucher

Hi I spent a certain amount of money over a period of time online and I was awarded a £20 voucher which was valid until the 6th of August aslong as I spent at least £60 or more which I placed a order on the 4th August for delivery on the 5th of August when I went to check out I tried to use the voucher which was still in date and it wouldn’t accept it I rang customer services and spoke to a Mohammed who asked about certain items in my basket which could cause this problem and we removed them still the voucher was coming up not valid and I had to pay full price for my goods at the checkout and he assured me it would get sorted since then I’ve rang numerous times sent emails and chats on the business chat and sent numerous pictures of the voucher and no one seems to help to sort this out I have found it totaly disgraceful that it’s now the 21st august and still nothing sorted I’m totaly dissapointed and dissatisfied with Morrisons I’ve shopped with yous for at least 20years and have lost total faith and exhausted with this whole experience