I own Minecraft but it still wants me to buy Minecraft

Here are some things to check:

• Double-check you are logging in to the same Microsoft account you used to migrate your Mojang account.
• Make sure you are not logging in with your Mojang account. Migrated accounts will only be able to log in with your Microsoft account.
• If you forgot which email you used to create your Microsoft account for migration you will need to speak with Microsoft support to recover the email address used to sign-up.
• You may not have completed your migration, please go to your Mojang account profile page and start the migration again.

Source: Microsoft Account Issues and FAQ