To link your Odoo Enterprise subscription code to your database and remove the expiration warning, follow these steps:

1. Go to Odoo Subscription Page – Log in to www.odoo.com/my/subscriptions and find your subscription code.
2. Access Your Odoo Database – Navigate to Settings > General Settings in your Odoo backend.
3. Enter Subscription Code – Scroll down to the Enterprise Contract section, enter your subscription code, and click Apply.
4. Refresh & Verify – Restart your Odoo instance or refresh the page to ensure the warning disappears.
5. Contact Odoo Support – If the issue persists, submit a ticket via www.odoo.com/help for assistance.

This should successfully link your subscription and remove the expiration notice. 🚀

You have to click on the link "register your subscription" and then enter the code in the field "Subscription code", and clic "Send".

As simple as that.

By the way, you can get help from Odoo at https://www.odoo.com/es_ES/help or https://www.odoo.com/es_ES/help-form 

activate developer mode 
go to the system parameter 
Add/update database.enterprise_code in key and code in value