The numpad is locked (NumLock is off): click the numpad-locked notice in the window, or press NumLock on your keyboard.
If you set numpad number keys as hotkeys and pressing them does nothing, the numpad is most likely locked (NumLock is off).
How to fix it
When the numpad is locked, a red notice saying the numpad is locked and you can click to unlock it appears above the player, to the left of Hotkeys (①). Click it to unlock, or press NumLock on your keyboard.
TipWhen setting a hotkey, NumLock also needs to be on for numpad keys to be recorded.
