@Clare76 

Does it happen when you connect the extended WiFi network, or when you login the extender via web interface or Tether App?

Please note that the default WiFi network of range extender is open, no password required, and after configuration, it will have the same wireless password as your router.

Regarding the login password, it is either admin by default, or created by yourself for the first time login, if admin doesn't work and no other possible password, please reset the extender to factory defaults by holding reset button/hole until all LEDs turn on momentarily.

  @Qummumbaban 

Could you please let us what kind of password do you mean?
1. If you mean the login password of your TP-Link device, it is suggested for you to use admin in lower case to check. However, if admin cannot work, please reset your TP-Link device to default settings by pressing and holding the reset button for 10s. 
2. If you mean the password of your TP-Link ID, please reset the password on the following web link:
https://www.tplinkcloud.com/forgotpassword.php
3. If you mean the WiFi password of your TP-Link extender, please note that its wifi password is the same as your router after you set it up, so please use the wifi password of your main router to have a try.