If you have an available product, you should find the offer here in your online Sonos account profile:

https://www.sonos.com/en-us/myaccount/user/offers/upgrade

I saw an offer and clicked on it but then ‍♀️. Now that link you shared doesn’t show an eligible product. I have a Beam Gen1 which the website says qualified 

If you add a new Sonos Beam (just as an example) to your shopping basket and goto the basket, do you see the voucher to select, prior to checkout? - I’m thinking you may have already upgraded and that it’s perhaps in your account and the discount voucher is already there (and waiting) to be used? I would just check that and if it’s not there, then go onto sort the matter with Sonos Sales, who can be contacted via this link:

https://support.sonos.com/s/global-sales-help

Thanks Ken. I wondered that, but no. There are some odd things happening in my account so maybe there’s an issue there. 
I’ll contact support using that link. Thanks for taking the time to answer