How to turn off advertising

It sounds as though you have 'Restrictions' turned on in Screen Time.

Limit Ad Tracking greyed out

1) Go to Settings

2) Tap General

3) Tap Screen Time

4) Tap Content & Privacy Restrictions

5) Turn on Content & Privacy Restrictions

6) Scroll down to Advertising and choose Allow

7) Tap Restrictions

8) Enter the passcode

9) Select Restrictions

10) Tap Allow Changes

Now go back to Settings > Privacy > Advertising and turn off Limit Ad Tracking.

Try reading it again:

1) Go to Settings

2) Tap General

3) Tap Screen Time

4) Tap Content & Privacy Restrictions

5) Turn on Content & Privacy Restrictions

6) Scroll down to Advertising and choose Allow

7) Tap Restrictions

8) Enter the passcode

9) Select Restrictions

10) Tap Allow Changes

Now go back to Settings > Privacy > Advertising and turn off Limit Ad Tracking.

Did you read seanachai51's last post at all?

What advertising?

That does not turn off Advertising, it turns off the limits to Ad Tracking. Do you wish to have advertising track you and your usage?

Have you side loaded an app from outside of the app store that installed a management profile?