A lot of question and answer on the internet and I found one that meets my case.
I installed new Ubuntu on my laptop and I choose a long long computer name, so my host name is so long and I wanted to change because it made the terminal so long and I could not concentrate with that so I searched for how to change host name and here is how:
sudo gedit /etc/hostname
then go to gedit and edit it to something shorter.
But since then Pidgin can not add Gtalk account. So the problem is the hostname and the hosts don’t meet. Open hosts and see this
127.0.0.1 [old hostname]
so I use
sudo gedit /etc/hosts
and edit from
127.0.0.1 [old hostname] --> 127.0.0.1 [new hostname]
now add Gtalk account to Pidgin without any problems.