Hello Guys, I have realized where this issue came from and here is the reason for reference if anyone faced the same issue.
I've reinstalled the frontaccounting from the beginning again with the same settings as before except that I've changed the default Database Script of chart of accounts to be "5 digit American coa with educational data" and the problem became a trouble because now the main company username and password didn't work at all. I spent sometime also for trying to solve it from the mysql backend by changing the md5 for the password and so on just to confirm that the type of database script selected is the issue.
Then I removed the whole frontaccounting and I started over but I chose different database script for the COA, then it solved.
The issue is from some database scripts so it's a good idea to start without educational data, but of course it will be quite hard to understand the system for the newbies including me. first time I use the frontaccounting.
So now I think will need to go through the documentation to see how to build my coa.
Hopefully this post will put the light on the issue for developers to fix it and to help others who might face the issue.
Thanks for everyone contributed with a comment. you helped me a lot guys. Highly appreciated.