how to recover restriction code

You forgot your Screen Time passcode?

If using IOS 13.4 or later or Catalina 10.15.4 or later, see the Apple Support article(s) below. Link to contact Apple Support at the end of the article.

Screen Time passcode reset.   *

ASC YouTube for how to reset your Screen Time passcode on iPhone, iPad, and iPod touch — Apple Support

If necessary,  you will 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.      *

See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

BTW: 

Check iPhone type & iOS version -> Go to: Settings -> General -> About.

If it is an iPhone 6 -> iOS 12.5.5 is the last security updates for iPhone 6, -> released 23 september 2021.

If the iPhone is still on iOS 11, update to iOS 12.5.5 

Go to: Settings -> General -> Software Update.