I bought Minecraft but it does not work. I only have the demo!
It can take up to 24 hours for the account to be fully active. But it is also there if you buy it.
Go on https://accounts.mojang.com/....com/login log in.
See if there Minecraft is listed as a game, if so everything is fine and it will take some time until everything is ge-sync-ed.
If not, wait until 24h again and then contact Mojang Support if the problem persists.
Did you buy the Bedrock or Java Edition?