@Sadiejuly 

Here are some suggestions you could give it a try for incorrect password error: 

Case1:  RE Led/ Wi-Fi LEDs are OFF on the range extender, and wireless client will not accept the password of router

It is possible that  wrong password has been put in during the RE set up, please factory reset RE and reinstall it from the beginning, put in correct Wi-Fi password during set up
Reset:with RE powered on, use a pin to push the reset hole for 10 secs until most LEDs flash once

Case 2: RE Led/ Wi-Fi LEDs are ON on the range extender, and wireless client will not accept the password of router

Download tp-link Tether app, double check Range extender's password on tether app and use that password to connect WiFi :
https://www.tp-link.com/support/faq/2541/

If you found WiFi password becomes very long digit random WiFi password, please let us know the model of your router, thank you.