Are you talking about Night Shift, to change the white balance of your screen to be more yellow, or Dark Mode, which can be turned on permanently without scheduling?
It sounds like you’re trying to schedule Dark Mode to stay on all the time, which can be accomplished by choosing “Dark” in System Settings-> Appearance in Ventura or later, or in System Preferences-> General in Monterey or earlier.
https://support.apple.com/en-ca/HT208976
