Hi there, this is Marc. Thanks for reaching out to the community.
The launcher shows “Buy” when it can’t detect the license tied to the account you’re signed in with. That means you’re logging in with a different Microsoft account than the one the purchase was bound to.
Go to https://account.microsoft.com/billing/orders and search your purchase history using the PayPal transaction date. That will show which Microsoft account the license is attached to. You must sign into the Minecraft Launcher with that exact Microsoft account, not Mojang credentials, not a secondary email.
If the order doesn’t show under your current account but you have the PayPal receipt, you’ll need to work with Microsoft Support and provide that PayPal transaction ID. Only they can trace the license to the correct account.
Best regards,