For the LED light, it has different status on different devices or different hardware versions. For your case, if the extender is TL-WA850RE_V1, it's normal when the wireless light is flashing, that means the wireless function is working properly. 

Please firstly reset the extender, then try to set it up again.

To set it up, if you are using TL-WA850RE_V1, we can only set it up via web interface, Here are some instructions for your reference.  

https://www.tp-link.com/en/support/faq/698/  

If the hardware version is V2 or later, we can use either web interface or Tether APP. Here are the instructions for Tether APP.

https://www.tp-link.com/en/support/faq/1373/