Hello, mates 
I can’t find the brightness slider to change the value. Only see the volume slider. I also saw the display settings and nothing there.
Someone suggests a solution, please?
What system do you have? What do you see under GNOME’s Power settings?
It should be here in the upper right corner where my mouse is. Just click on it to bring up the options.
If this is not what you see, please screenshot and post. Edit, apparently no mouse. either way, it’s the half full bar under volume.
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If you are on a desktop or have a docked monitor, I don’t think Gnome will let you control an external monitor’s settings.
Otherwise, do check Settings > Power: there is usually a slider there.
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Yes, it should, but, unfortunately, it is not.
See:
I can’t find it in the settings either:
Do I have to install anything?
Yes, I’m using a desktop. Is that the problem?
I am a little blind and that’s why it would be good some light 
But, there is no way to skirt round this problem? Installing any extension, for instance?
change on monitor. no extension i know.
" If you have increased the brightness to the maximum but it’s still not bright enough, you may need to adjust the contrast or gamma settings of the screen instead. These controls will also be found on the monitor or in one of its menus, although not all monitors have them. " 
Edit . above from gnome (help) 
It explain clear word !
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You could try xbacklight or light to do it from a terminal, else what @Shjim said.
light -S 100 # Sets to 100%
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I’m not sure if there’s even some API to change brightness on a non-integrated monitor. I usually manage it from the monitor’s physical controls.
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Have you ever been able to alter the brightness on an external monitor using software on your PC? With any OS?
Probably not, given that tends to be something that a laptop is capable of due to its built-in hardware.
However, you may find success with gddccontrol (in the AUR).
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Ok, boys.
Thank you very much for all help.
My stepson helped me changing directly on monitor.
Thank you very much!
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Sincerely, I don’t remember. I started using a desktop recently. I worked with desktops in offices with MS-Windows, of course, but at home I just had laptops. And recently (3 or 4 months ago) I decided to buy a desktop and put aside my laptops. And that’s why sometimes I feel like something is missing. Perhaps you cannot believe me, but some days ago I was still looking for the battery icon 
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