so you don't have master page settings in your site settings. Then you need to activate the publishing feature as below
Activate publishing at the site collection level
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From the root of your site collection, click Settings
> Site settings.
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On the Site Settings page under Site Collection Administration, click
Site collection features.
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On the Site Features page, scroll down to SharePoint Server Publishing Infrastructure and click
Activate.
Once publishing features are activated, subsites inherit them automatically. But, if you built a site before activating the publishing infrastructure, youll have to manually activate it for each site.
Activate publishing at the site level
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From any page within the site, click (gear icon) Settings
> Site settings.
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On the Site Settings page, in the Site Actions section, click Manage site features.
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On the Site Features page, next to SharePoint Server Publishing, click
Activate.