Assuming that Pinnacle has been installed properly on your computer, the most likely reason that you are unable to log in is that you are using an incorrect login ID or password. You may need to check with your system administrator to make sure that these are correct.
Pinnacle will not allow you to log in more than once, from different computers for example. If you are logged in to Pinnacle on one computer, you cannot change to a different computer and log in a second time.
Other possible reasons for not being able to log in are:
There is something wrong with the way Pinnacle has been set up on your computer.
You are trying to connect to a database on a network and the network database engine is not running.
In each case, you should contact your system administrator for help with rectifying the problem.