Macbook Pro Mid2012 with OS Sierra 10.12.6 won't download apps without 10.13.6 or later. No update available on AppStore
Logic Pro X requires macOS High Sierra 10.13.6 or later. You'd need to upgrade the MacBook Pro from macOS Sierra 10.12.6 to macOS High Sierra 10.13.6 or later.

Your model of MacBook Pro (Mid 2012) supports the latest macOS available as of this writing, which is macOS Catalina 10.15.4.

So here's what you need to do to be able to install and Logic Pro X.


Use this link to download macOS High Sierra 10.13.6 installer from the Mac App Store.
Once the installer downloads, you can run it to upgrade your system to macOS High Sierra 10.13.6. It's advisable to keep a backup of the system/important data files before you begin the installation.
Once you have the OS upgrade installed, you can install Logic Pro X from the Mac App Store.