Hi Alex_N, in general there was “Plundervolt” around 2020 when every vendor started locking this in their UEFI/Bioses. For example I own HP Elitebook 840 G3 and I had to downgrade BIOS to Version/Date HP N75 Ver. 01.42, 19. 8. 2019 ( before 2020 ) to be able to change voltages (for example via throttlestop).
So try to downgrade to older bios.
In your case - HP Elitebook 840 G5 you can try bios somewhere around ver. 01.09.01 or older.
L.