Hi Mustafa,
Welcome to our community! 
Do you use an antivirus program or a firewall that might be blocking Unity? If not, try to restart the installation. Maybe there is/was a technical issue either on your side or on Unity’s server.
If you cannot install the documentation, don’t install it and/or try it later again. You can find it online:
I hope this helped.
See also:
- Forum User Guides : How to apply code formatting within your post
- Forum User Guides : How to mark a topic as solved