Do you use the plugin through your ATS? Then you have to sign in with your ATS account rather than with a NIXZ login.
First check whether you are signed in to the correct ATS environment and whether the NIXZ Plugin is activated for your account. If you still cannot open the plugin, ask your ATS administrator whether the connection is set up correctly.
If you use a direct NIXZ environment with dashboard access, check whether your username and password are correct and reset your password if needed.