@Mohsen-1 

Hello,

Did you mean that you can't connect to the range extender's wifi after configuration or you can't configure it successfully?

What is the Error information? May I have a photo of it?

You can try to reconfigure it.

Please download "Tether app" on your cell phone from APP store/ Google play, and refer to this FAQ to install Range Extender on Tether app: 

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

If you can't set up the RE450 successfully, please refer to this FAQ to do the troubleshooting:

https://www.tp-link.com/en/support/faq/2234/

If you need any further help, please let us know which step you met trouble, the Error message, and send us a photo of LEDs on your range extender, thank you!

Have a nice day!