@gcc 

If your main Wi-Fi has no password, but requires a username and password on a webpage to log in in order to get internet like a public Wi-Fi then the extender will not work.

If your Wi-Fi is just a typical router at your residence that has no wireless password set up for it, the extender should be able to connect to that. 

What you could try is to set it up over the web browser: https://www.tp-link.com/us/support/faq/1411/