Screen time passcode

Then you will have to erase the device as instructed. If you restore from a backup made when the code was applied it will just re-apply the code. I know you won't like having to do it that way but if it all one had to do was backup the phone, erase it, then restore the backup the whole idea of a restrictions passcoe would be meaningless and millions of kids would be resetting their devices to get past parental restrictions.

I am not clear on your actual question. Disable Screen Time or change the settings to enable you to make purchases. If you forgot the passcode:

Click on the link to read what to do "If you forgot the [restrictions or ScreenTime passcode] for your iPhone, iPad, or iPod touch, or your device is disabled" - http://support.apple.com/HT204306 - You can restore from a backup but that backup has to date from before you set the restrictions/Screen Time passcode.

You will have to disable those restrictions using the settings for the feature that set them, e.g., Screen Time

Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support