I phone forgotten screen time password

Follow the instructions in this link-->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support. When prompted set it up as a new phone; do not restore a backup. The Screen Time passcode is stored in the backup and you will be back where you started if you restore a backup that was made after the Screen Time passcode was enabled.

The only way to clear it if you can't remember it is to erase the device using Find My iPhone or by forcing it into recovery mode and restoring iOS using iTunes.

After you erase it, do NOT restore your backup. The restriction settings are contained the the backup. If you restore from a backup made after you set that passcode, you'll end up right back where you started.