How do you remove a screentime passcode if you've forgotten it?

You will have to do a Restore. Also, see Use Screen Time on your iPhone, iPad, or iPod touch.

Restore:  Connect your iPhone to iTunes on your computer, backup, and then select Restore to Factory. Also, see Restore your iPhone, iPad, or iPod to factory settings and If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple.

For help with the above on later models, including iPhone 7, 7s, 8 or later, iPad Pro 11" and iPad Pro 12.9" models, please refer to If your iPhone, iPad, or iPod touch won't turn on or is frozen.

If all else fails then make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. If you did not purchase it from Apple then you will need to look to the vendor for service or How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

When you erased the phone, did you restore it from a backup made when the Screen Time restrictions were in place? If you did, you restored the restrictions.

You need to restore the device to factory settings. Do not restore from a backup made when the Screen Time code was active.

Restore to Factory setting

Parental controls

Parental Controls and Family Sharing

You have to restore as New, otherwise, it will restore all your old settings including the passcode. This is like forgetting a Restrictions passcode.

Forgotten Restrictions Passcode Help

  •   iPad,iPod,iPod Touch Recovery Mode

  •   How to erase your iOS device & then set it up as a new device or restore it from backups

  •   About Restrictions (parental controls) on iPhone, iPad, and iPod touch - Apple Support

  •   Use parental controls on your iPhone, iPad, and iPod touch - Apple Support

You will need to restore your device as New to remove a Restrictions passcode. Go through the normal process to restore your device, but when you see the options to restore as New or from a backup, be sure to choose New. You can restore from a backup only if you have one from BEFORE you set the restrictions passcode.

The option will appear after the system restore when you will be prompted if you want to restore a backup or as New. The procedure is outlined in the link, How to erase your iOS device & then set it up as a new device or restore it from backups.