As expressed earlier in this forum, the User that is trying to access Data Protector does not have the correct permissions. It could also be a hostname or IP address resolution issue
The way to track this down
ON your Linux cell manager edit the file
/etc/opt/omni/server/users/UserList
and add this line, temporarily, to create a user with wide-open permissions
"*" "*" "*" * "admin"
There is 1 space between each field. Save the file, and try to connect again. If it connects, close the GUI, and re-edit the UserList file to add the hostname of the Windows 2012 system
"*" "*" "*" hostname_2012 "admin" forexample
Save the file, try to start the GUI again. If it starts, then close it, and in the 2nd field, add the Username that you are using to log into the Windows 2012 system. The Username must be upper-case characters for a Windows user
"*" "SHINIX" "*" hostname_2012 "admin" for example
DO NOT, under any circumstances, leave the '*' in the 4th field, this is a huge security rick
If adding the hostname for the Windows 2012 is not useful, then, you must edit the 'hosts' file on the W2012 system to add the IP address and fully-qualified domain name. I'm not sure where the 'hosts' file would be on Windows 2012, but, on previous version fo Windows, it was at:
Windows -> System32 -> drivers -> etc -> hosts