I assume you are not in the UK? If not, SmartThings may not offer it as an option in regions outside the UK. You should contact ST support at https://support.smartthings.com
You can just change the region to US, I leave in Europe and I moved my account to US in order to have everything available.
So far I have:
ST HUB V3 purchased from US
Amazon Alexa purchased from US
Philips HUE Hub + Lightstrip purchased from Europe
Harmony Hub purchased from US
Ikea bulbs purchased from EU
Motion sensors, water leak sensors, arrival sensors purchased from US