Advertising Identifier

If you have restrictions set then it will be greyed out because there are no ads limit (go to settings>Screen Time>Content & privacy restrictions>advertising - if this is on "don't allow" then limit ad tracking will be greyed out.

It will also be greyed out if you have a business or school email account on your phone, if you are under 13, or you have a phone under Device Management (meaning a phone issued by an employer).

But why do you want advertisers to track everything you do on your phone? If it's to get a free game you are selling your soul for a lot less than it's worth.