It is impossible to activate active track.
The situation is as described by this video I found on forums and attach here:

https://www.youtube.com/shorts/iIzePAJcL6c

What is the solution to be able to tick the dot and make it active?

Thanks for the answer

Dirty Bird Posted at 1-15 09:58
I believe that is the toggle to activate Subject Scanning.  The drone has to be airborne for the toggle to be selected.  Likewise you must be airborne to activate ActiveTrack or any of the other Intelligent Flight Modes.

*Note that the toggle only activates Subject Scanning & displays a small green + on trackable subjects.  It can be annoying if there are many people or cars in frame as the screen will be full of many + icons.  The easiest way to engage ActiveTrack is just draw a small box around the desired target with your finger.

Hi, thanks for the reply but I WAS AIRBORNE. Drone was hoovering 5 meters on my head :-)

Hello there. Thank you for reaching out. Please check the camera settings. The "Subject Scanning" function only works under single shoot or video mode with 30fps and below. Please keep us posted. Have a nice day.

Hi there. Thank you for reaching out and we're sorry for the trouble. Can you please try to perform the ActiveTrack outdoors and check if the issue persists? Make sure to enable the subject scanning via the DJI Fly app. You may refer to the conditions below on how the DJI Mini 3 Pro ActiveTrack works:

If a person is set as a tracking subject, the distance between the aircraft and the subject should be 4 to 20 m, and the altitude is 2 to 20 m. (Note: The recommended distance is 5 to 10 m and the recommended altitude is 2 to 10 m.)

If a vehicle/boat is set as a tracking subject, the distance between the aircraft and the subject should be 6 to 100 m, and the altitude is 6 to 100 m. (Note: The recommended distance is 20 to 50 m and the recommended altitude is 10 to 50 m.)

If the aircraft starts to follow the subject, but the distance between the aircraft and target is not in the range of abovementioned distance or altitude, the aircraft will automatically fly into the supporting range. If the aircraft flies in the range of recommended distance and altitude, you will have a better experience of tracking.

If the issue persists, please download the DJI Assistant 2 software on your computer and please update/refresh the firmware of your unit then make sure that you are using the latest version of the DJI Fly app. You may provide us a screen recording showing the issue outdoors if you still experience the same problem.