Everything is okay now.
At first, I thought that my laptop may have some problems so I tried using my friend's laptop but not successful.
After that I changed the Internet system I was using and used another computer. I was successful.
(But when I tried again with my laptop in this Internet system, it still remained this error. I dont understand why it is that)
If you get such a error, you should try with another computer in another Internet system.
Thanks so much for your help.
I will take my test on October 25th.
Good luck, friend!