I see from your image that it is not a login problem; it is "creating demo account with this broker" problem.You can go to the broker's website and create an account directly from them.
You should try to make the login by searching for the server again.
Of course, make sure you're not trying to access a Demo Account with live credentials and vice-versa.
Obviously all of this should be checked after making sure your connection is working.
here's where you can open a demo account on FBS.