Delete all the lines after 127.0.0.1 linux.server linux, just in case (or move this line to the very end). Save the file. (Use vi editor as root).

su root
vi /etc/hosts

Now restart Oracle services. If you still see issues please provide me the output of the following file:

cat /etc/sysconfig/network

Hasan,
this time when I enter the vi /etc/hosts command I got this warning:

E325: ATTENTION
Found a swap file by the name "/etc/.hosts.swp"
owned by: root dated: Sun Jul 7 13:54:02 2013
file name: /etc/hosts
modified: no
user name: root host name: linux.server
process ID: 6366
While opening file "/etc/hosts"
dated: Thu Jun 13 14:27:07 2013

(1) Another program may be editing the same file.
If this is the case, be careful not to end up with two
different instances of the same file when making changes.
Quit, or continue with caution.

(2) An edit session for this file crashed.
If this is the case, use ":recover" or "vim -r /etc/hosts"
to recover the changes (see ":help recovery").
If you did this already, delete the swap file "/etc/.hosts.swp"
to avoid this message.
"/etc/hosts" 2L, 110C
Press ENTER or type command to continue

I press the "enter", it showed the same message as above.

sorry I cannot find the cat /etc/sysconfig etwork.
Can you show me more the detailed steps to get it please?

Hasan,
I tried this command: cat /etc/sysconfig/network.
this is the output:
[Oracle@linux ~]$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=linux.server

is this what you're looking for?
I really appreciate your kind help!

Yes please take this line to the very end please to rule out any hosts file related issues.

"127.0.0.1 linux.server linux"

Enter the line without quotes.
127.0.0.1 linux.server linux

Hansan, Can you show me how to do that? Do I have to use any command? Because I can't copy, paste or do anything on the screen after I did vi /etc/hosts and got the output:
127.0.0.1 linux.server linux
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
~
~
~
~
~
~
~
~
~
~
~
~


I cannot do anything after all these ~ ~ ~

Hansan,
So I remove everything after "127.0.0.1".
I start installing Oracle EBS again. this time I got 2 red on the post installed check.
Port Availability
File Systems.

I attache the files for you.
please help!

port availability file