1. Please follow this installation process: General installation process
    - ComposerName: order-attribute
    - ModuleName: OrderAttribute
    - If you're installing manually,  the archive's file content has to be uploaded  to: app/code/Magetrend/OrderAttribute/

The extension is not working without the license key. If you don't have it or don't know yours, please follow this article:

To install the license key, please go to:

Stores >> Configuration >> Magetrend Extensions >> Order Attribute>> Information & License

Enter the license key, and press "Save Config" button.
Note: please use development license key if have installed extension not in product store.