Hello Cristina,
You should first install and activate the Lorina Pro theme as the license key is not available for the free version.
After installing and activating the paid version, you will see the Theme License section at Appearance > Theme License.
Lorina and Lorina Pro are regarded as separate themes in the WordPress setup. If you have made changes with the Customizer when using Lorina (free version), you can copy/import your customizations from Lorina to Lorina Pro with one click using the ‘Theme Mods’ section that is available in Lorina Pro.