Warning: Shipping method required! ERROR!

I have a serious problem. After I have passed the shipping option on the payment page, I am going to go back to the shipping address section and when I want to go forward, it gives a cargo selection error and the stage can not passed. Please help me.

https://www.vookadin.com/kasa

TEST USER LOGIN: test@test.com
PASSWORD: test

Just click to "devam" button to go to last step and go back to shipping adress section and try to go to the last step again. You'll see the error on shipping methot section. There write "Warning: Shipping method required!"

Please help me this is very important for us.
Thank yoy
Best Regards

you did not provide the url of the site

you are right sorry I added the link to first message