@Ruben-06 

You mean wireless password or login password of the web interface?

For wireless password, after configuring the extender successfully, it shares the same wireless password as your main router

For login password of the web interface, it is either admin by default or created by user manually when configuring it at the first time, if admin doesn't work and you are unable to remember other possible password, please reset the extender to factory defaults and set it up again.