What is power mode: https://docs.microsoft.com/en-us/windows-hardware/customize/desktop/customize-power-slider
It seems, when you have missing power schemes in the registry, the combobox will be hidden: Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\User\PowerSchemes
There should be power schemes for each (balanced, energy saver, ...) where the friendlyname have the word "Overlay".
Easiest way to fix this is to export "Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power" from a working computer and import it to on the broken computer. After restart, the slider comes back.