Hey @qsxie
Range extender will allow you to create a new login account after factory reset the extender. Some older version extender use ' admin' as username and 'admin' as default login password. You could factory reset extender if login failed
RESET: With the extender powered on, use a pin to press the RESET button until most Leds flash once.
Later please follow the instructions here to set up range extender :
(How to configure Range Extender via Tether APP)https://www.tp-link.com/support/faq/357/
(How to configure my range extender via web GUI to extend an existing wireless network's coverage)https://www.tp-link.com/support/faq/1400/
May I have a photo of the error if the concerns persists, thank you.