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I am facing this issue that, it's disabled
You are currently in the site settings interface of a top-level site, the top-level site doesn’t have a parent site so it can't display the same navigation items as a subsite site.
You can enable this option in a subsite. If you are in a subsite, it will display as below:
For Site collection does not have the parent, so navigation will not inherit from parent. But in the subsite this option must be enabled because subsite have the parent site
Required, but never shown