On some newer versions of Raspberry Pi OS the Fan service fails to start. Luckily these newer version of the OS have a built in fan control you can turn on.
In command prompt or terminal connect to the pi using SSH and run the command:
Select Performance Options
The default GPIO pin is 14, but the BrainCraft Hat has the fan connected to pin 4.
You can set the temperature to anything between 60-120 degrees Celsius. 80 is a good midway point in that range.
You'll need to reboot after changing the settings.
Now your fan will come on when ever the board is over 80 degrees. To check the current temperature of your board you use this command: