You could try it with adding this kernel parameter:
I read that it also solved brightness control problems on imacs. If it doesn't help, repeat all steps and remove the added term from the file instead.
I'm not sure why you don't have a slider for the brightness. Can you adapt the brightness at your monitor?
Are you on XOrg (X11) or Wayland?You can test with
echo "$XDG_SESSION_TYPE"
If the output shows Wayland, logout and at login click on your username then a cog wheal appears at the bottom right corner. There select "Zorin Desktop on Xorg"
When you are on XOrg, you can set the brightness also with xrandr. Enter xrandr in terminal to find out the name of your monitor and then you can set a value for the brightness between 0 and 1
xrandr --output <outputname> --brightness 0.8
https://www.baeldung.com/linux/monitor-brightness-change
An alternative could be the installation of brightness-controller which is easier to use:
Is secure boot disabled in your BIOS?