How can I enable the field, 'Do not Contact', for the layouts duplicated from standard layout?

By default, the Do Not Contact field isn't enabled in the layouts created from the standard layout. You can add this field to the layout so that your users can see and use this in the side panel.

To add the field, go to the sandbox of your sales application. Open the CX Microsoft 365 Designer (Sales application > Application Composer > Productivity Applications Setup > CX Microsoft 365 Designer). From the menu (hamburger icon), open standard layout of your Contacts page. From the Available Fields, drag-and-drop Do Not Contact. Save your changes and publish.