When you are unable to access the lock screen with the PIN, it usually ask for the Gmail account attached to the device to continue. The last option is hard resetting the device.
Thanks. As a last resort, we recommend you to perform a Hard reset. This will delete all personal information (pictures, text messages, etc.) and third-party apps saved on the phone. This will also erase the lock screen you have set up on the device. Please, make sure you remember the Gmail account, and the password before starting the process. Can we proceed?