The clone button is only available when the default theme is eligible for cloning and the correct theme management page is being used. Make sure you're logged in with an administrator account and check Design → Themes (or the equivalent theme management section) rather than the storefront/theme editor.

Also, the instructions may be for a specific AbanteCart version, so the UI can differ between versions. If the Clone button is completely missing, check your AbanteCart version and whether the default theme is installed/enabled.

If you share your AbanteCart version and a screenshot of the Themes page, someone should be able to pinpoint what's different.