The possible reasons are as follows:
1. It may be that the WiFi password is really entered wrong, it is recommended to check the input.
2. It may be that the input method used by the iPhone6 is not clear about the case of the password, so it is necessary to distinguish the case of the letters when entering.
3. If the iPhone is not turned off for a long time, too much data may also occur, you can restart the phone after shutdown and then enter.
4. Check whether the set WiFi password has been changed before entering.