Hi Shirin,
You have three problems that need to be fixed. If you go to Extension Manager and look at Warnings (screenshot attached) you will see that the tmp directory is not writeable and you have a memory limit problem. You can fix the memory limit problem using this tutorial.
www.ostraining.com/blog/joomla/installation-problems/ The second problem is the permission on the configuatation.php file. It's best on a live site to set this to 444 for security, but while you're making these changes set it to 644.
Third thing is to triple check the location of your tmp folder. You've got two warnings that relate to that. The problem with the location may be fixed after you change the permission on configuration.php. Even though you are typing it in global configuration, it may not be getting saved because the path doesn't get written to the file.
Try these fixes and see if it makes a difference.
Ed