Screen time

If you’re on an iPhone 6, that’s iOS 12 and earlier, then you will want to factory reset the iPhone, and not restore a backup, as the backup will restore the passcode. (You can try Apple Support too, as they might have a tool or procedure that can reset the passcode without resetting the iPhone.)

For newer versions: If you forgot your Screen Time passcode - Apple Support

fortuneneuman wrote:

It’s asking for restriction passcode and I can’t remember it

That does not change the previous answer.

There’s no means to reset the screen time passcode as far back as an iPhone 6, so you’ll have to factory reset the device, and not restore, assuming Apple doesn’t have a tool for this now.

Check with Apple.