That means you're entering the wrong PIN / Admin PIN / passphrase, depending on what it's asking you to enter. The default PIN is 123456 and the default Admin PIN is 12345678. The passphrase is something you set during generation of the key, so I can't help you there. If you locked out the applet, you may need to reset it to return it to default -
https://developers.yubico.com/ykneo-ope ... pplet.html (skip the prerequisites section since you're using the YubiKey 4, this is only if you have an old YubiKey NEO)