Hi ChrisDingman1,
Thank you for reaching Microsoft Community. I understand that you are having difficulties in using the Smart Charging feature of your Surface Pro 9 and also, after disabling the option, the Smart Charging feature disappears from the Surface app as well.
For Smart Charging, users only have the option to disable the feature whenever they need to have the device charge up to 100%. In enabling the feature back however, it will only be the device that would turn on the feature automatically whenever it detects that it needs to enable the feature. Basically, the feature is always active even if the user turns it off and turns on automatically when it detects your device is plugged in for prolonged periods or used at elevated temperatures. See this page for more details: Smart charging on Surface - Microsoft Support
For the disappearance of the Smart Charging section on the Surface app though, kindly try first updating the application from the Microsoft Store (Get updates for apps and games in Microsoft Store - Microsoft Support) and make sure all Windows updates are installed including Optional updates in Windows Settings > Windows updates. Run also the Surface Diagnostic Toolkit on the device to run some system checks and repairs then see if the Smart Charging section would then show up again in the Surface app. See this page for reference: Fix common Surface problems using the Surface app and Surface Diagnostic Toolkit - Microsoft Support.
Let me know how it goes.
Kind regards,
Marrion