How can I recover restriction passcode

You can't. You will have to erase the phone either using Find My iPhone or by forcing it into recovery mode and restoring iOS using iTunes.

After doing so, set it up as a new phone. Do NOT restore a backup made after the restrictions were enabled or you'll end up right back where you started.

You can't recover your restrictions passcode. If it were recoverable it would be a useless feature. You will have to restore your phone and set it up as New, without restoring your backup, because the Restrictions/Screen Time passcode is included in the backup. See this Apple support link-->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Now that I have posted the party line, there may be ways to find out what the passcode is. A carefully constructed web search may turn up a method or three.