It is undoubtedly asking for your Lock Screen passcode. If you use FaceID or TouchID, you also have to have a 6-digit passcode that allows to you enter your device if there is a problem with either the facial or touch recognition. Whatever you supplied for that passcode at the time should be the code you need.
The other possibility is that it is your iCloud Keychain passcode (I don't use Keychain, so I'm a bit in the dark here). But is it possible that it wants your iCloud Keychain passcode?
Best,
GB