iCloud 1TB storage not available after purchase

• When did you buy it?
• Are you sure it was successful?
• Got email confirmation?
• Does rebooting the device help?

Check the icloud storage on another device then where you purchased the storage. Is it showing up there?

Where did you bought it?

I was looking into some support articles about iCloud+ and the available storage plans and 1TB is not offered

iCloud+ plans and pricing - Apple Support

Did you go through the official Apple channels?