If you have purchased Minecraft, and have an account token, but have not yet redeemed it for a username, that option will be highlighted and available. Scenario: You purchased Minecraft, or you redeemed a gift code, but you didn’t select a username.
Solution: Log fully out of the launcher with your account, and then log back in again. Doing so updates the account’s status, and you'll be able to access the full version. Scenario: You purchased Minecraft with a Mojang account while that account was logged into the launcher. The following are additional scenarios where you would be able to play Minecraft only in demo mode.
You do not need to download or purchase the game again unless you received an email stating that you were refunded.
If you would like to try Minecraft: Java Edition before deciding to purchase the game, download the Minecraft launcher.