From --> 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 who are under age 18 by birth year, signed in with their Apple Account*
If your Apple Account is managed by an educational or business institution
If your device is managed and uses a configuration profile that limits tracking
If your Apple Account was created in the last 3 days"
Note it says by birth year. Even if you are 18, if it is going by year then the year could still be making it think you are 17.
Read this support article about how to update the birth date associated with an Apple ID: Update the birth date associated with your Apple Account - Apple Support