@Hoje 

The extender needs the wireless password that you normally use to connect to the routers Wi-Fi. If you do not remember what is it, you can get on a device that is currently connected to your router and log into it, go to it's wireless settings and grab it there. The extender does not use the routers password that you would need to log into the settings.

Here is a FAQ to help with the setup: https://www.tp-link.com/us/support/faq/1373/

I have the same problem. The extender is set up, and all the lights except ethernet are lit so it thinks it is connected to the router. It shows up as an available wifi network on my devices (have tried two) and they will attempt to connect, but the normal router password is rejected as incorrect. I have entered it many times and checked it visually on both devices.

I downloaded the Tether app and looked at the device properties there. It gave a different password--very long and in hexadecimal, and I have successfully logged on with that.

@Hoje 

when using my modem default password, it generated a 63 character password for the extender.  When changing the modem default password to one of my choosing, it assigned that password to the extender.

@Wifian 

when using my modem default password, it generated a 63 character password for the extender.  When changing the modem default password to one of my choosing, it assigned that password to the extender.