You could try the advice in these threads

http://www.gog.com/en/forum/spellforce_platinum/cannot_start_game

or

http://www.gog.com/en/forum/spellforce_platinum/desired_resolution_not_found_solutions_wont_work

In the folder where your installed the game there is a file called "config.lua". Open this file with Notepad and look for "ScreenResolution = " Try changing the value until you can get the game to run. I'd suggest starting with 0 and then 1, etc

When the game starts up, you can use the game's interface to change the resolution.

Remember that when you change your resolution the window bar might pop up so you might have to experiment to get a perfect fit. I think I had about 1650x1020 where as my native is 1680x1050 and it was perfect, filled up the whole screen and I put the window bar to auto-hide which made the perfect fullscreen solution. This was probably with window mode as with fullscreen something would happen. This is on Win7 64-bit.