Hi there,
sorry to hear that you are having an issue with Screentime on your iPhone.
First let me begin by advising you that the screentime can only be activated by someone who has physical access to the device. Apple does not have any way to activate screentime on your device.
When screentime is setup on the phone, a passcode needs to be created to ensure that only you can access your Screentime settings. This also ensures that parents are able to manage and monitor their child's device without the child being able to modify the settings that the parent has applied.
Screentime was introduced in iOS12 and replaced restrictions which was available on iOS11 and earlier. If restrictions was enabled on ios11 before you upgraded to iOS12 then screentime would have been activated automatically.
The only way to remove a restrictions passcode on a device is to restore the device to factory settings.
Restriction passcodes are also included as part of the backup when the backup is created so you will be unable to restore a backup of your data as it will reapply the screentime restrictions. This is designed so those that have the screentime restrictions added for a reason (parents for kids or employers for employees), they are unable to remove the restrictions without wiping everything.
Apple is not able to disable the restrictions for you, the only way is to remember the passcode or reset the device.
Regards,
Rob C.