@K502SOHEL 

Could you please let us know what kind of password do you mean?
1. If you mean the login password of your TP-Link device, it is recommended to try admin in lower case. However, if admin doesn't work and no other possible password, please reset your TP-Link device to default settings by pressing and holding the reset button for 10s. 

2. If you mean the Wi-Fi password of your TP-Link device:
For range extender, please use the Wi-Fi password of your main router to have a try, and it should be exactly the same as your main router.
For TP-Link router, please use the default Wi-Fi password to see if it works. The default Wi-Fi password is printed on the label of this device itself as wireless password/PIN. If the default password doesn't work, please reset your device to factory defaults and try again.