The laptop which you have purchased has had its screen replaced with a higher end 1920x1080 display due to the item being refurbished. Because this is a higher resolution display, the brightness functionality will not work on a hardware level. You can try using software such as CareUEyes Lite which is a free program that allows for screen dimming on a software level. I've included a link to it below.
https://care-eyes.com/careueyes-lite/?buy