I bought Minecraft a long time ago, can't go into realms. Says I need to buy MC again.

I bought Minecraft in the 2012-2014 times, and I've been wanting to play Realms with some friends. I tried to go on it but it just came up with:

 
Mojang implements certain procedures to help protect children and their privacy, including complying with the Children's Online Privacy Protection Act (COPPA)
 __ 
You may need to obtain parental consent before accessing your Realms account.
If you have an older Minecraft account (you log in with your username). you need to migrate to a Mojang account in order to access Realms.
 
And below that there is an "Update Account" button. But whenever I press it, it sends me to the "Buy Minecraft" page. And I've already bought Minecraft so I don't know how that is happening.
If anyone in the Mojang/Microsoft team could help, I'd gladly appreciate it.
 
Thanks.