Why is my new lenovo tab 8 saying authentication problem can't connect to Wi-Fi?

Authentication problem means bad password.
connect to your router directly and check the password for Wi-Fi. Maybe your neighbors changed it.

It may have had an update which affects the way it implements the security algorithm. Simply FORGET the signal and then Re-Join it(make it recreate your security credentials by typing in the PassKey value again) even though a passkey may not change, the generation of the security credentials it contributes to can be changed by updates in some cases. If you are entering your passkey and its not working then make sure you are connecting to YOUR Wi-Fi and not someone elses. If necessary do a Wi-Fi channel scan using a mobile app to see what channels are occupied by Wi-Fi signals.