I had the same problem, they never sent email and we extremely unhelpful when i called, telling me I had to 'find the email myself' and that it was probably in my junk. They wouldn't confirm the sender address and told me i came from a third party. I suspect it is a total con! Very disappointed with the service.
I had a look into your account and cashback email was sent to you back in September 2019. Code from this email is valid for 60 days so unfortunately this cashback offer has now expired.
You can find more details about Cashback offers here
Please accept apologies for this inconvenience.