I installed Zorin on an old iMac without any issues.
But I noticed the screen brightness was low and when I went to the control center to adjust the brightness as per Zorin troubleshooting I found there is no Brightness slider.
I tried installing the Zorin with Nvidia drivers but it failed.
How or can I get the Brightness slider to appear in the Control Center?
Thanks
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?
Which nvidia drivers are shown when you take a look at Software&Updates > Additional drivers?
No NVidia driver show in the Additional Drivers list.
I have tried X11 and the slider does not appear in the control center.
I am using Zorin on a 2012 iMac and do not have access to the BIOS.
X11 turns not on the slider but is better for use with nvidia cards.
Probably you need 390 legacy driver for your nvidia card. You could try it with Zorin 17. But the brightness control should be independent whether you use nouveau or the proprietary nvidia drivers.
The GTX 675MX card is legacy and has no support for drivers later than the 390 set.
The 390 set is not supported in later kernels - including for Zorin OS 18.
Likely, the 5.15 kernel is the latest you can use, which is not offered in the Zorin OS 18 repository.
The options this leaves you with include using an older release of Zorin OS, or doing a bit of configuring and swapping of the kernel and drivers for Zorin OS 18.
EDIT:
It seems great minds think alike at the same time. 
Is 390 installable on noble? It seems not to be in the repository of noble (empty).
I think the 390 driver can only be used with Zorin 17.
From what I just read, it sounds like its probably best, for the user to run on Zorin OS 17, using the 5:15 kernel, which would allow activation of the 390 Nvidia driver, which in turn, will make the OP happy.
Great news for the OP, there is still 2-years of official support left for OS 17, so yay for them. 
Or iamliving can use the default 6.8 kernel of Zorin 17, add the kelebek333 ppa and then install the 390 nvidia driver of the ppa.
