Create Apex classes in a production organization

What You Should Know About Apex in Production Orgs

• Unlimited Edition and Enterprise Edition Production Organizations can execute Apex and view Apex in the user interface, but modifying Apex in the user interface is not allowed.
• Apex classes and triggers must first be written in a Developer Edition, Sandbox, or Trial Organization, along with appropriate test methods achieving at least 75% code coverage.
• To deploy Apex to a live Production environment, use either the Salesforce ANT Migration Tool or the Salesforce Extensions for VS Code (formerly Apex Toolkit for Eclipse).
• The New button does not appear in the Apex Class section of Setup in Production, and the same applies to Apex Triggers.

Note: You can only create new triggers from within an Object's setup page. There is no "New" button on the Apex Triggers section in the Setup menu in Production.