can't we login to GUI interface of LTM using root user name

The root account is used for CLI access only.

Refer to the following knowledge base article for descriptions of default F5 accounts:

https://support.f5.com/csp/article/K13148

Hello Thiyagu,

Root is by default a maintenance account designed for Command line interface. You have an account called admin for GUI with same privilege.

Hello Thiyagu,

Only the user root can have login for CLI, the user admin can have access for GUI and CLI, this configuration can do in system/user and change the access of the user admin with advances shell, or can you integrate F5 with different programs of authentication, for example active directory or TACACs or Radius.