Hello @Ruth8
TP-Link range extender will use the same Wi-Fi password as the router after being configured successfully, this can't be changed.
If the wifi password changed to a very long random password, we have a thread discussing on the same topic, please try some suggestions in Case 3:
[Troubleshooting] Unable to Connect to WiFi Extender Saying Incorrect Password
May I know the model of router, the firmware version of the router? Did you use WPS button / Web page/ Tether app to set up RE?
What is the firmware version & hardware version of your WA854RE, can I have a photo of the very long password ? Thank you
How to find the hardware and firmware version of my TP-Link device?