Android shows “Authentication error”, or “Authentication problem”, when the phone cannot prove to the router that it knows the Wi-Fi password. The handshake fails before any internet is involved, so the fix is almost always about the password or the saved network, not the internet connection.
The fix, in order
Re-enter the password carefully. Wi-Fi passwords are case sensitive. Check for a capital letter, a number, or a symbol that is easy to mistype, and use the show password option while typing.
Forget the network and reconnect. In Wi-Fi settings, tap the network, choose Forget, then select it again and enter the password from scratch. A saved profile from before a password change is the most common cause.
Check the password is current. If the router password was changed recently, every device needs the new one. Confirm the current password in the router settings or with the person who manages the router.
Restart the phone and the router. A stuck authentication state on either side clears with a restart.
Check the router security type. Very old devices may not support WPA3-only networks, and some routers fail with mixed WPA2/WPA3 modes. Switching the router to WPA2 is the practical workaround for an older phone.
Check the phone's date and time. A badly wrong clock can break authentication on some networks, so set date and time to automatic.
Never enter your Wi-Fi password on a website or in an app that claims it can recover or test it. The password only belongs in the device's own Wi-Fi settings.
If it still happens
If the same password works on other devices but this phone still fails after forgetting the network, try connecting the phone to a different Wi-Fi network. If it fails everywhere, the phone's Wi-Fi radio or software is at fault, so update the phone and, as a last step, reset its network settings.
Common questions
What does a Wi-Fi authentication error mean? It means the router rejected the phone's login attempt, almost always because the password is wrong or the saved network details are out of date. It is not an internet outage.
Why does it say authentication error when the password is right? The saved profile may hold an old password, the router may have changed its security type, or the password may have a character that was typed differently. Forgetting the network and typing the password again fixes most of these.