Click on this link to learn what it means 'If an app asks to track your activity' --> If an app asks to track your activity - Apple Support
"In some circumstances, the "Allow Apps to Request to Track" setting can't be turned on, including:
For users with child accounts or under age 18 by birth year, signed in with their Apple ID*"
Make sure you have the correct birthday set. If you recently turned 18, try logging out of your account and back in again as suggested in this topic --> App Tracking Age Requirements Issue - Apple Community
If you wish to ask Apple about this:
Choose your country or region - Official Apple Support