Hi Kaye HLY,
Thank you for reaching Microsoft Community. Apologies for the inconvenience this has caused you.
Does the back light turns off only when you leave the typecover idle for a few seconds or it really turns off while you are using it?
Does this happen also when using the device while plugged in or not?
If the backlight turns off only when you leave the typecover idle for a few seconds, this is a one of feature of the Surface to save more power but back light should light up automatically when you press a key. You don't need to toggle the backlight again like pressing the designated key just to turn it back on.
Please try going to Device Manager, expand Keyboard then see if Surface Type Cover Filter Device is present. If yes, double-click on it to view its Properties. Select Power Management tab if available. Uncheck the box next to Allow the computer to turn off this device to save power. Click OK then restart your Surface.
Try also going to Windows Settings > System > Power > Power mode then set it to Best performance and test if still the typecover back light turns off on its own.
Kind regards,
Marrion