The following article describes two possible ways to solve missing Access Request Settings from Site Settings » Site Permissions.
The solution applies to: SharePoint Server 2013, SharePoint Server 2016, SharePoint Server 2019, SharePoint Online
Missing Outgoing Email Settings
(applies only to SharePoint Server)
When you navigate to the Site Settings > Site permissions and see the following view:
Verify Outgoing Email Settings in Central Administration:
Check out the following articles for possible configurations:
Configure outgoing email for a SharePoint 2013 farm
SharePoint 2016 Outgoing Email Configuration settings
Inherited Permissions
(applies to both SharePoint Server and SharePoint Online)
The Access Request Setting is absent when the subsite inherits permissions from its parent:
Stop inheriting permissions and the setting becomes available:
Results:
